# B-17G with a 20 mm cannon onboard!



## seesul (Jul 1, 2008)

One of 8 B-17´s shot down on August 29, 1944 in our area had a 20 mm cannon onboard! B-17G, serial # 42-31885,MACR8099, crash site Vyskovec.
A friend of mine found 20 mm shells at the crash site last year, then we noted a strange gun on the historical picture (attached) and this week I´ve found out this cannon still exist (pic attached, taken from Fragmenty z B-17G – Detektor web.cz detektory kovù!!!

More at 20 mm cannon aboard a B-17G???

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## FLYBOYJ (Jul 1, 2008)

Wild stuff Roman - this is from the site you referenced...

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## Milos Sijacki (Jul 1, 2008)

This is something completely new for me. That thing probably recoiled so much that it actually didn't do any good.


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## comiso90 (Jul 1, 2008)

Definition of helpless:
Sitting still in a B-17 armed with .50 cals and while a German fighter closes in popping off 20mm rounds and waiting till your .50s are in range.

arrgghhh!

.


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## renrich (Jul 1, 2008)

Comis, I think you would find that a 50 BMG has a longer effective range than most 20 mms although not as destructive with a hit.

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## comiso90 (Jul 1, 2008)

I wonder where I picked that up then?..
seems like i heard a B-17 gunner saying it..
oh well thanks for the correction.


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## seesul (Jul 2, 2008)

renrich said:


> Comis, I think you would find that a 50 BMG has a longer effective range than most 20 mms although not as destructive with a hit.



Renrich,

this is what you can find at mentioned discussion at ArmyAirForces.com posted by Martyjhawk:

Hi Roman,
From what I have read and been told, the few planes with the 20-mm guns flew the tail end charlie positions in their formations because the planes in this position were often attacked by fighters with guns of greater range than the 50's normally installed in the tail position. The fighters could lag behind the bombers and shoot at them, but the bombers lacked guns of effective range to shoot back...


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## seesul (Jul 2, 2008)

FLYBOYJ said:


> Wild stuff Roman - this is from the site you referenced...



Joe,

we believe this cannon was mounted in tail. This aircraft was the last in the formation...I know this version did exist, but don´t have any pics. Don´t you have any?


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## FLYBOYJ (Jul 2, 2008)

seesul said:


> Joe,
> 
> we believe this cannon was mounted in tail. This aircraft was the last in the formation...I know this version did exist, but don´t have any pics. Don´t you have any?



I have no pics of this Roman but it would seem to me despite the photo of the nose mounted cannon, the tail would seem like the best place. I'll look around for some more information.


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## seesul (Jul 2, 2008)

FLYBOYJ said:


> I have no pics of this Roman but it would seem to me despite the photo of the nose mounted cannon, the tail would seem like the best place. I'll look around for some more information.



This is what I´ve found on Internet:
- 42-38090 340BS/97BG 'MISS WINDY CITY' was one of five B17Gs from 97th BG which had a similar 20mm cannon fitted in their tails
- The 99th BG also had at least one B-17 with the 20mm gun in the tail. Aircraft was 42-32046, LIL ABNER, according to Dick Drain's 5th Wing History of Aircraft Assigned.
- According to Roger Freeman's book The B-17 Flying Fortress Story, B-17's with these tail cannon were flown at the rear of formations, and the weapon was considered more of psychological than destructive value.'....

Anyway, the mentioned B-17G, 42-31885, was transfered to 2ndBG from 99thBG, see the copy taken from the database at Second Bomb Group
Assigned to 99th BG Feb 44. Transfer to 2nd BG 28 Mar 44 after 14 missions. MIA Moravska Ostrova 29 Aug 44 on its 49th mission, total 63. Attacked by fighters, crashed near Vsetin. Merrill Prentice crew, Heath, McVey, Laux, Goldstein, Ellis, Balcerzak, Johnson, Petrey, Fitch. McVey POW, rest KIA.


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## seesul (Jul 2, 2008)

FLYBOYJ said:


> I have no pics of this Roman but it would seem to me despite the photo of the nose mounted cannon, the tail would seem like the best place. I'll look around for some more information.



A friend of mine is going to scan the pics with the tail cannon mounting and send them to me. I´ll post them as soon as I get them...


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## renrich (Jul 2, 2008)

If I was a gunner with a pair of 50 mgs confronted by a fighter with a pair of 20 mms, I would probably feel overmatched. I doubt that the gunners were too familiar with ballistic tables or the ballistic properties of the 50 BMG. In the Pacific the Wildcats liked the headon shot against the Zero not only because they had a more rugged fighter but because the flat trajectory of their 50s let them open fire before the Zero pilot had a good chance of hitting with a more rainbow trajectory of his 20 mm Oerlikons.


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## FLYBOYJ (Jul 2, 2008)

Thanks Roman, I'd love to see them.


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## Erich (Jul 2, 2008)

Roman the range in this case for aerial combat of the 20mm oerlikon (sp ?) was just plain terrible to the effectiveness of the .50cal besides being lighter they could of course rain out more destructive power than the US 20mm cannon - heavy, slow to operate and probably just a plain nuisance and not even having the overall range by using a heavier hand held weapon.

E ~


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## seesul (Jul 2, 2008)

Erich said:


> Roman the range in this case for aerial combat of the 20mm oerlikon (sp ?) was just plain terrible to the effectiveness of the .50cal besides being lighter they could of course rain out more destructive power than the US 20mm cannon - heavy, slow to operate and probably just a plain nuisance and not even having the overall range by using a heavier hand held weapon.
> 
> E ~



yes, as they say:''the weapon was considered more of psychological than destructive value"
btw, have you ever seen a pic of B-17 with that Hispano in tail?
I´d like to know how many B-17´s had this instalation...


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## Erich (Jul 2, 2008)

no Roman I have not, the experimental --------- is it YB-40 supposed to be an heavy fighter killer equipped with over 17 mg and cannons. I may have the designation incorrect - circa 1943


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## seesul (Jul 2, 2008)

Erich said:


> no Roman I have not, the experimental --------- is it YB-40 supposed to be an heavy fighter killer equipped with over 17 mg and cannons. I may have the designation incorrect - circa 1943



yes, I know YB-40, they were too heavy and slow, lagged behind the formation...


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## Erich (Jul 2, 2008)

to be truthful Roman I think they were complete fantasy and never flew in action, a believed myth by single-engine LW fighter pilots during mid-war.

by the way ~ nice signature ~

E ~


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## Micdrow (Jul 2, 2008)

Here is a picture of a YB-40 for those that do not know what it looked like. Though if I remember right none of them had the 20mm tail cannon. Not to get off subject but some may find the bottom picture is of interest also. Dont remember where I got it but if I remeber right the B-17 was modified around 1949. It was a failure because of the movement of the wing which made the gunner more subsitable to air sickness.


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## HoHun (Jul 2, 2008)

Hi Renrich,

>Comis, I think you would find that a 50 BMG has a longer effective range than most 20 mms although not as destructive with a hit.

I once asked Tony Williams (author of "Rapid Fire" and the "Flying Guns" books) for the formal definition of the term "maximum effective range". To my surprise, he pointed out that it was a rather poorly defined term without a clear way to arrive at the figures you usually find quoted under this headline.

This has some relevance to the question of comparable effective ranges as we have to establish what it is what we'd like to compare 

I believe that the mention of "effectivness" in the term is decisive for a definition. It's not "efficiency", which would mean we'd have to compare effort spent to results achieved, but plain effect - results achieved regardless of the means.

If I'd make an attempt at a definition, it would be something like:

"Maximum effective range is the range at which a battery of guns, spending the full amount of ammunition that it has available, has a chance of destroying its target with a certain probability in an engagement of a certain duration."

The probability mentioned in that definition should be comparatively low so that a target beyond maximum effective range could feel fairly safe from destruction - one might say 1% for example. (It's assumed that destructiveness decreases with increasing range.)

This definition is interesting because it leads to a number of unusual conclusions:

- Maximum effective range depends on

... the number of barrels installed,

... the amount of ammunition carried,

... the destructive power of the round,

... the durability of the target,

... the duration of the engagement.

Of course, it also depends on the "traditional" characteristics of a gun, such as hit probability at the range in question, but the unusual aspect is that maximum effective range is not a property of the gun alone, but rather one of the entire weapons system. For example, if one has already used up half of one's original ammunition supply, the maximum effective range drops for the rest of the mission.

I think this definition can be used to explain why a rear gunner can feel out-ranged by attacking fighters even if his rearward-firing 12.7 mm machine gun has a much flatter trajectory than the fighter's forward firing 20 mm cannon. 

(It can also be used to explain how Sorley arrived at the conclusion that the RAF fighters would need an eight-gun battery 

By the way, the Luftwaffe often used cannon in forward firing positions even when the rear guns were rifle-calibre only because combat experience showed that these guns would have only very short periods during which they could engage the enemy. The greater effect from cannon shells was more important than the added weight of the gun - this could be compensated by supplying it with a smaller number of (heavier) rounds.

I figure the idea behind the B-17 nose cannon was similar!

Regards,

Henning (HoHun)


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## Erich (Jul 2, 2008)

Ho-Hun your last remark was to counter the forward attacks in staffel strength by the Luftwaffe, but that was changed by summer of 44 due to the over increase of flight time through the bomber formation from the front with all US heavies mgs blasting anything that came through the line. The LW switched due that it would be easier to just confront the tail section only of the B-24/B-17 masses


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## HoHun (Jul 2, 2008)

Hi Micdrow,

>Not to get off subject but some may find the bottom picture is of interest also. 

Hm, I'd like to point out that for readers using Opera, there is no bottom picture but both are in the same line. This causes posts with multiple pictures to become very hard to read as the post (in fact, all of the posts on the current page of the thread) can easily become several times as wide as the user's screen.

This is (in my opinion) not a bug in Opera, but in the forum template as it separates the images with the   HTML entity which is a "non-breakable space" - it announces "do not insert a line break between the adjacent elements".

MSIE doesn't give a damn and inserts the line break anyway, obviously resulting in the easy-to-read layout the designer of the forum template intended.

If the   entity would be replaced by an ordinary space, or even by a <br> tag, Opera would display the thread the same way as MSIE, and that's probably the way the designer meant it to be displayed.

Since you're a moderator here and have a good technical understanding, I thought maybe if I'd point it out to you, you might know which administrator to contact for a possible fix  I once made a post in the suggestions/comments subforum, unfortunately without receiving an answer:

http://www.ww2aircraft.net/forum/co...sts-attachments-simple-improvement-12757.html

I guess my post appeared too technical over there :-/ But it's really an inconvenience that hits me at least once every day, and often in the most interesting threads where people have posted great high-resolution pictures, such as in the current Do 335 thread - or this fascinating thread on B-17 armament, of course! 

Regards,

Henning (HoHun)


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## Micdrow (Jul 2, 2008)

Hi Henning, 

As to your question on the board I would say you would have to contact evengilder or horse. I know next to nothing about the message board or how they work. In fact I usually have to go to evengilder ever time I have a problem. Most of the stuff you wrote about the message board idea is well like speaking a foreign language to me.

My main roll as a moderator is to allow me to make the technical area data base and to be able to move files around more easily. 

On a side note the B-17 is probably one of my most favorate aircraft. After this weekend maybe I will start a technial thread about the aircraft and some of its different configurations. Currently Im trying to orginize all my data since I seem to have a scatter mess right now.


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## renrich (Jul 2, 2008)

I don't think there is any mystery to max effective range. I don't have my ballistic tables handy but there is something called the rule of three. With a 3006, 270, 308 class of cartridge, sighting in to be 3 inches high at 100 yards will mean that the bullet will be within 3 inches of point of aim anywhere out to 300 yards. On a deer size target that means that a shot within 300 yards is essentially point blank and the shooter does not have to worry about range or bullet trajectory. A 50 BMG has a ballistic coefficient much higher than a 308 or 270 bullet and it's muzzle velocity is similar so point blank range on a target such as an airplane would be much further than 300 yards. Some 20 mms are not going to have as much MV as the 50 BMG and the projectile of the 20 mm is going to have a substantially lower BC so the 20mm is not going to be as flat shooting. Of course the more curved trajectory of the 20 mm will be compensated for by the bore sighting and gun sight of the fighter. I believe the bottom line is that the 50 BMG would have somewhat of an advantage in making a hit at longer ranges but that would be off set by the greater destructive power of the 20 mm shell. If both AC were firing at 300 yards or less, the 20 mm would have an advantage.


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## Glider (Jul 2, 2008)

Re the installation of the 20mm in the rear of the B17 I admit to not knowing how on earth they fitted it in. The rear gun position of a B17 is very small and if you look at the arc of fire of even the HMG, it was pretty limited.
A 20mm is massive compared to the .50 and how they managed to get anyone inside to aim let alone reload the thing is beyond me.

You would stand a better chance of getting it in the rear of the B24 if you replaced the rear turret with a manual position


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## drgondog (Jul 2, 2008)

Glider said:


> Re the installation of the 20mm in the rear of the B17 I admit to not knowing how on earth they fitted it in. The rear gun position of a B17 is very small and if you look at the arc of fire of even the HMG, it was pretty limited.
> A 20mm is massive compared to the .50 and how they managed to get anyone inside to aim let alone reload the thing is beyond me.
> 
> You would stand a better chance of getting it in the rear of the B24 if you replaced the rear turret with a manual position



I'm with you Glider. I have sat in that stupid bicycle seat for both the Cheyenne and the 'original' tail gun position. I have serious doubts that a single 20mm could be mounted as the breech was much longer than the M2.

I am ware of the Nose mount - that was feasible from a dimension perspective.

Seecil - the YB40 flew ~ 10-15 combat missions. It's only redeeming quality was the chin mount that converted to the B-17G. It kept up going in - but every body else lost 5000 pounds at the target and it didn't.

Eric is right about the tactics change - for at least two reasons. In March 1944 with the increase in P-51 Groups more Sweeps were plannned out in front, and secondly - the heavy armament of the Fw 190A8 made more rounds on target available from rear - and it was easier to find and stalk a bomber wing from the rear. Pilot skills requirements to shoot a B-17 at closing speeds of 500+ head on were far higher than 200-250mph from 6 O'clock.


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## seesul (Jul 3, 2008)

Micdrow said:


> On a side note the B-17 is probably one of my most favorate aircraft. After this weekend maybe I will start a technial thread about the aircraft and some of its different configurations. Currently Im trying to orginize all my data since I seem to have a scatter mess right now.



WOW, CAN´T WAIT TO SEE IT!!!


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## seesul (Jul 3, 2008)

Glider said:


> Re the installation of the 20mm in the rear of the B17 I admit to not knowing how on earth they fitted it in. The rear gun position of a B17 is very small and if you look at the arc of fire of even the HMG, it was pretty limited.
> A 20mm is massive compared to the .50 and how they managed to get anyone inside to aim let alone reload the thing is beyond me.
> 
> You would stand a better chance of getting it in the rear of the B24 if you replaced the rear turret with a manual position




There´s really no space for the tail installation but I´ve found few mentions on Internet about it (see above) and also friend of mine told me there´s a pic of it in Freeman´s book...he promised to scan it so as soon as I have it I´ll post it...


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## HoHun (Jul 3, 2008)

Hi Micdrow,

>As to your question on the board I would say you would have to contact evengilder or horse. 

Thanks, I'll do that! 

Regards,

Henning (HoHun)


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## HoHun (Jul 3, 2008)

Hi Renrich,

>I don't think there is any mystery to max effective range. I don't have my ballistic tables handy but there is something called the rule of three. 

Hm, I see what you mean, but it's my impression such a kind of well-defined rule is not the basis for the usually quoted maximum errective range figures for aircraft armament. Or Tony Williams simply didn't know about it 

>On a deer size target that means that a shot within 300 yards is essentially point blank and the shooter does not have to worry about range or bullet trajectory. 

That's a good point - I don't know if you remember the generic patterns "as viewed through the gunsight of a fighter" I posted a while back, but I based these on just this idea of point blank shooting: http://www.ww2aircraft.net/forum/weapons-systems-tech/centerline-guns-configurations-11037.html 

(For wing-mounted fighter guns, the lateral deviation from the aim point usually has a larger impact than the vertical one, by the way.)

If we're talking about rapid fire weapon, it might even be necessary to include the pattern size into the consideration as fully automatic fire from airframe-mounted batteries tends to be far less accurate than aimed single shots from a handgun. For the tail turret of the B-17, the dispersion circle was given as 25 mils, meaning that the pattern would spread over a 10-m-wingspan fighter from wingtip to wingtip at 400 m range. Of course, the fighter in a head-on aspect will only fill a fraction of the pattern "disk" so that your hit chances are down to a fraction, too - even if the centre of the pattern perfectly coincedes with your aim point.

Accordingly, I'd say a big part of the problem for the B-17 gunner would be that the fighter with its fixed and considerably more rigid gun mounts would have a considerably smaller dispersion (a 4 to 6 mil circle for the MG 151/20) then the bomber's defensive guns.

For example, at 800 m range, a fighter's pattern (for example from the wing root guns - the outer wing guns would not be worth much beyond convergence range) would have less than 5 m diameter. Even with just two 20 mm cannon firing, bringing that pattern to bear on the bomber would do very serious damage.

The bomber gunner on the other hand would have a pattern with 20 m diameter, meaning that the fighter would only fill a small fraction of the pattern - and that most shots would miss even if the gunner aims perfectly. If we assume that the frontal area of the fighter fills something like 2% of the pattern, we might get 3% hits for the sake of the example as the pattern is denser towards the centre, and that means firing 100 rounds he'd get just 3 hits randomly distributed over the Focke-Wulf - which in the same time could 184 rounds at the bomber, of which (depending on the placement of the pattern) several dozen could hit if the aim is right.

So the gunner would have reason to feel out-ranged, even though the trajectory of his 12.7 mm machine gun is flatter than that of the 20 mm MG 151/20 of his opponent ...

Regards,

Henning (HoHun)


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## Soren (Jul 3, 2008)

The MG151/20 fires a higher SD round at 770 m/s, and therefore it should have a comparable trajectory to the .50 BMG.


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## renrich (Jul 3, 2008)

Henning, I understand that some or maybe all of US wing mounted guns in fighters were mounted in a manner where only one point on the gun was solidly fixed to the airplane. I think that means that the barrel was free to move about with some amount of motion. Can you explain that a little more clearly and how that would affect bullet dispersion. A high SD does not necessarily translate to a high BC.


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## Glider (Jul 3, 2008)

Soren said:


> The MG151/20 fires a higher SD round at 770 m/s, and therefore it should have a comparable trajectory to the .50 BMG.



I admit that my understanding was that the .50 had a MV of around 890m/s and had a better trajectory. The 20mm having far more impact at longer ranges due to the explosive content of the shell, but not due to the trajectory.


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## Erich (Jul 3, 2008)

yes the 2cm was effective IF the LW pilot could get into range before being hammered by .50's of the bombers first, time and again in books and through personal interviews the Fw 190/Bf 109G crews mention getting assailed by .50 rounds before they could get their two to four 2cm to bear during rear attacks. of course the slow firing short range 3cm was even a worse case scenario


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## HoHun (Jul 3, 2008)

Hi Renrich,

>Can you explain that a little more clearly and how that would affect bullet dispersion. 

Hm, my point about lateral distance from the aim point was one of simple geometry - both short of and beyond convergence distance, even the idealized trajectory that is not subject to dispersion does not coincede with the aim point.

Bullet dispersion in a wing mount is affected by vibration of the structure it's mounted to, and wings tend to be more flexible than structural members of the fuselage.

Of course, it depends on the actual aircraft in question as well as on the gun mount, but as a rule of thumb wing guns that are mounted outside the propeller disk can be considered to have roughly 50% more dispersion than fuselage mounted guns.

Barrel vibration is also a major source of dispersion - the Luftwaffe found the low-velocity MK 108 to be more capable than the high-velocity MK 103 due to its combination of high rate of fire and small dispersion. High-velocity weapons pay for their flatter trajectory with an increased dispersion, which was one of the reasons the 50 mm aircraft cannon didn't work out for the Luftwaffe.

Regards,

Henning (HoHun)


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## Erich (Jul 3, 2008)

the MK 108 was used due to the aerodynamics and non dampening effect/vibration of the wings in the Fw 190, the longer barrel was not suitable of the MK 103 except for ground attack work on twin engines where it proved itself


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## Soren (Jul 4, 2008)

Glider said:


> I admit that my understanding was that the .50 had a MV of around 890m/s and had a better trajectory. The 20mm having far more impact at longer ranges due to the explosive content of the shell, but not due to the trajectory.



That's also true. However firing AP rounds (Not exactly the std.) the MG151/20 should have a comparable trajectory to the .50 BMG. With the lighter and less aerodynamic HE(M) shells the trajectory was worse than the .50 BMG, that's true.


Another thing is that a bomber is a larger target to be shooting at, and the fighter is a more stable gun platform than firing from a mount on the bomber. So for that reason the effective range of the fire from the fighters was longer. Now that doesn't mean that their guns fired at a flatter trajectory, just that they could land their bullet closer together, and since their target was larger they could expect to hit at longer ranges. A small, fast and nimble fighter is a tricky target for a gunner to hit, eventhough he can shoot accurately out to 1,000y at a stationary target the same size.

Still as Erich notes the US gunners were firing at the LW fighters at ranges where they werent even in what theyd call accurate firing range themselves, but neither were the US gunners, they were just throwing bullets at the LW a/c hoping to get some hits. Now despite what many think a single .50cal hit can do a whole lot of damage to an a/c. At 1,000y its still got enough power to tear through an engine block, which wouldn't be so nice if your a/c was fitted with a watercooled inline engine!


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## drgondog (Jul 4, 2008)

Soren said:


> Another thing is that a bomber is a larger target to be shooting at, and the fighter is a more stable gun platform than firing from a mount on the bomber.
> 
> *Soren - how do we know this to be true? A twin 50 mount in the tail of a B-17 is not only 'pretty stable' but also flexible to the extent that the Gunner can compensate for a/c motion.
> 
> ...



I'm not really challenging the statements - just wondering how they could be proved


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## seesul (Jul 4, 2008)

Still waitin´ for my friends scans of the 20 mm cannon instalation on tail of B-17.
Does anyone have these pics?


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## renrich (Jul 5, 2008)

It is my understanding based on comments made by Linnekin in "80 Knots tp Mach 2" that the guns in the wings of WW2 US fighters jumped around a bit because the mountings were not rigid. He stated that the vibration when firing 6-50s at 600 rpm was very noticeable. It would seem to me that if the muzzle is moving perhaps one half inch or more the dispersion would be pretty major down range. This would be in contrast to bomber mountings which would be more akin to a tripod mount used by infantry or pedestal mount on tanks.


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## Soren (Jul 5, 2008)

> A second question to be asked, particularly for wing mounts on an Fw 190, is why do we belive that the wings are 'stiffer' under high aero loads, to the point that the convergences are close to ground calibration?



Unless G's are being pulled there shouldn't be any difference than when firing on the ground, and they are more stable platforms, eventhough they undoubtedly vibrated to some extend when fire was commenced. Hence why the Zerstörres used to lob shells at the bombers out of reach from their defensive armaments.

And then there is again the fact that a bomber is a larger target which doesn't move all over the place, while a fighter is much smaller. 

Now I have no argument with the fact that the gun mounts on bomber were stable and good platforms, infact I've had this debate before with Adler where I was the one argueing for them. But I also recognize its a different deal shooting at a stationary target on the ground and then have to shoot at a moving fighter from a bomber in the air.


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## HoHun (Jul 5, 2008)

Hi Renrich,

>This would be in contrast to bomber mountings which would be more akin to a tripod mount used by infantry or pedestal mount on tanks.

Hm, here is a page reproduced from a WW2 manual showing the dispersions encountered when shooting from different gun positions of a B-17 bomber.

The dispersion strongly varies between different mountings, and for some reason the B-17 tail turret is the least accurate mounting (except for the even more inaccurate hand-held tail guns).

For for 12.7 mm Browning M2 machine guns in wing mounts, I'd suggest an approximate dispersion figure of 12 mils, and this is roughly what you see for the upper and chin turrets. The ball turret is even more accurate.

Regards,

Henning (HoHun)


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## Glider (Jul 5, 2008)

Silly question coming, but what is a tail stinger in a B17? 
Also I am more than a little surprised that the tail gun is worse than the waist guns, which have always struck me as pretty poor.


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## seesul (Jul 5, 2008)

Glider said:


> Silly question coming, but what is a tail stinger in a B17?
> Also I am more than a little surprised that the tail gun is worse than the waist guns, which have always struck me as pretty poor.



Think Stinger (aka Steplechase) was the original tail ''turret''. Later replaced by ''Cheyenne'' turret. Liberty Belle you´ve seen today has Cheyenne perhabs...
Something about it perhabs here Turrets


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## Soren (Jul 5, 2008)

Gotta second what HoHun says..


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## Tony Williams (Jul 6, 2008)

The concept of "effective range" is indeed a complex one as it depends on the ammunition, the weapon, the user and the target.

Take .30 cal guns, for instance. A hunting rifle was mentioned before; with this, you want a quick clean kill so your bullet must strike in a small vital area. So the effective range is quite short - around 300m maximum as suggested (for a good shot - probably 150-200m for most shooters). Use the same weapon in a military force and your intention is to inflict casualties, you don't care so much where you hit them: effective range perhaps 800m (which is also the effective range of a sniper firing a .30 cal rifle). Chamber the same .30 ammo in a medium machine gun fitted with extended-range sights and you can perform area-denial fire out to around 4,000m. You won't hit any specific targets at that range, but you'll drop the rounds within an area which will discourage the enemy from using it, so the fire is still effective.

Let's turn to aircraft. I would define "effective fire" as that which will destroy, or inflict severe damage on, the target aircraft. Clearly, the longer the range, the less effective the fire will be, because projectile dispersion plus the trajectory curve and the lengthening time of flight will reduce the hit probability, plus the effectiveness of MG bullet strikes will be reduced as their velocity drops (this does not much affect explosive cannon shells).

Other things beng equal, the bigger the plane, the easier it is to hit but the more damage you must inflict to bring it down. So the effective range of any particular armament depends on what you're shooting at.

In the Battle of Britain the RAF found that even with eight .303 MGs the maximum effective range against the Luftwaffe medium bombers was around 225m, because to bring them down with the little bullets required heavy, concentrated fire. 

The Luftwaffe discovered that to shoot down the tough B-17 required, on average, so much 20mm fire (allowing for the fact that only 2-5% of the shots fired hit the target) that the effective range of their fighter guns was very short, because they had to get very close to ensure enough hits. That's why they switched to the low-velocity 30mm MK 108 late in the war. On paper, the poor trajectory and long flight time of the 30mm shells gave them a short range, but in practice their devastating effect on the target meant that few hits were needed, so hit probability could be low (i.e. they could fire at long range). As a matter of interest, the Luftwaffe found the low-velocity MK 108 more effective than the high-velocity 30mm MK 103 at any given range, because for the same weight a plane could carry two MK 108 (throwing out 1,200 rounds per minute) for every MK 103 (around 400 rpm), and putting three times as many shells in the air gave a better hit probability than with the high-velocity gun.

The US fighters found that long-range fire could be effective against Japanese planes because they were so fragile and easy to set alight (until late in the war) that very few hits would usually do the job. 

For any given armament and target, the longest effective range was achieved in a head-on attack, because the target was flying towards the bullets and shells coming his way. An effective tail attack had to be carried out at a shorter distance because the target was flying away from the danger. Here, the rear gunner in a bomber had the advantage because the attacking fighters were flying towards him (but the disadvantage, as others have noted, of a far smaller target to aim at). The effective range in beam attacks was short simply because of the difficulty of calculating the correct lead angle for aiming, so the hit probability was very low. The introduction of gyro sights late in the war made deflection shooting much easier, so the effective range of beam attacks increased considerably without any other changes being made to the armament.

As you will have gathered from all of this, there are so many variables affecting the concept of "maximum effective range" that generalisations aren't a lot of use. All I can say is that judging by all of the pilots' accounts and official analyses I've read, only a tiny percentage of the aircraft destroyed in aerial combat were at ranges of more than 400m. Most kills were made at less than 250m. In a fighter vs fighter dogfight, with aircraft twisting and turning, the effective range was probably less than 100m.


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## seesul (Jul 9, 2008)

Back to B-17...a friend of mine got copies scanned and sent the to me.
Pics and text attached.
I really don´t know how this cannon could be mounted there as the bigger part of the barrel is inside the tail
There was no space for the tail gunner...


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## Bigxiko (Jul 9, 2008)

seesul said:


> Back to B-17...a friend of mine got copies scanned and sent the to me.
> Pics and text attached.
> I really don´t know how this cannon could be mounted there as the bigger part of the barrel is inside the tail
> There was no space for the tail gunner...



                  
I totally agree with you, those things are huge, those are not machine guns, those are real cannons, a Messerschmitz would be totaly wrecred if hitted with some of those .50
the tail gunner would never fit in there, and i believe neither the side gunners


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## Flyboy2 (Jul 9, 2008)

Dude the second scans are upside down!
Anyways.... i couldn't really read the first page. What were that nose gun mod


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## Erich (Jul 9, 2008)

just to add to Tony's ~ bottom of his posting, the 3cm Mk 108 fitted to the SturmFw's meant upon orders and their tactics the Fw 190A had to be within 100 yards to use the cannon effectively one reason the pilots slowed down and tailed down to fire into the fuselage and into the wing root. Opening up at first with the 2cm MG 151/20's at 400-600 yards to take out the tail gunner and then close to lethal range with the 3cm, hoping the waist gunners or other bombers in the box would not pummel the LW pilot to vaporization.

it had already been proven in December 43 and in January 44 with the Mk 108 in Fw 190A-6's of Sturmstaffel 1 that beyond 100 yards the Mk 108 3cm rounds would miss the target over 85 % of the time, in part due to closure rate as the pilots would not decrease their speed. Now of course this was slightly changed as the tactics were re-done. 

hopeful you will not find this a degression of the obvious present topic at hand - that of the 20mm mounting in the Fort

Roman your scan does show probably a pure experimental device and un-maned but in a fixed position fired by the cockpit crew.......


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## seesul (Jul 9, 2008)

Flyboy2 said:


> Dude the second scans are upside down!
> Anyways.... i couldn't really read the first page. What were that nose gun mod



Sorry, I didn´t scan it, just resent it.
Anyway, acrobat reader allows you to turn it back..


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## seesul (Jul 9, 2008)

Erich said:


> just to add to Tony's ~ bottom of his posting, the 3cm Mk 108 fitted to the SturmFw's meant upon orders and their tactics the Fw 190A had to be within 100 yards to use the cannon effectively one reason the pilots slowed down and tailed down to fire into the fuselage and into the wing root. Opening up at first with the 2cm MG 151/20's at 400-600 yards to take out the tail gunner and then close to lethal range with the 3cm, hoping the waist gunners or other bombers in the box would not pummel the LW pilot to vaporization.
> 
> it had already been proven in December 43 and in January 44 with the Mk 108 in Fw 190A-6's of Sturmstaffel 1 that beyond 100 yards the Mk 108 3cm rounds would miss the target over 85 % of the time, in part due to closure rate as the pilots would not decrease their speed. Now of course this was slightly changed as the tactics were re-done.
> 
> ...



Eric,

compare the lenght of the barrel on the pic taken on the ground (short lenght) and during the flight (longer lenght). It was manned by the tail gunner. One of 2nd BG vets confirmed to me...


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## Erich (Jul 9, 2008)

interesting. I assumed since I have talked with a couple local B-17 crewmen of the 8th AF in my neighborhood, and yes the 20mm Oerk. was field tested but found way to heavy, could not man it/turn quick enough to follow a LW fighter on a firing pass - used at the waist position(s). As you said about the tail though a tight fit, the recoil alone would just about break the shoulders of the tail gunner crouched in behind the cannon - and as said by many of us - what room ?


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## seesul (Jul 10, 2008)

Erich said:


> interesting. I assumed since I have talked with a couple local B-17 crewmen of the 8th AF in my neighborhood, and yes the 20mm Oerk. was field tested but found way to heavy, could not man it/turn quick enough to follow a LW fighter on a firing pass - used at the waist position(s). As you said about the tail though a tight fit, the recoil alone would just about break the shoulders of the tail gunner crouched in behind the cannon - and as said by many of us - what room ?


Yep, that´s really mystery...
One more thing, there were very likely 2 machines equiped with that cannon in tail. I mean 2 machines from the 20th Sqdn shot down in our area. Friend of mine found the 20 mm shell also on another crash site. It was B-17G ''My Baby'', 42-31473, MACR 8109.
The fact that both crews were complete also with their tail gunners support the fact that the tail cannon was manned by tail gunner. Now I don´t have my books on hand but later I can post the names and positions of those crew members. As far as I remember there was only one survivor from the first machine (where the pics come from), navigator Charles McVey, but he died few years ago in Chicago...And there were I guess 3 or 4 survivors from the other machine (My Baby) so I´ll try to find out if someone from them is living...


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## Glider (Jul 10, 2008)

Changing the subject a little I liked the picture of the 6 x .3 Browning chin turret. Interesting idea.


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## Micdrow (Jul 10, 2008)

Glider said:


> Changing the subject a little I liked the picture of the 6 x .3 Browning chin turret. Interesting idea.



From what Ive read on that one is it was a one off if I remember right made by an engineering ground crew to help with forward defense


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## HoHun (Jul 10, 2008)

Hi Tony,

>The Luftwaffe discovered that to shoot down the tough B-17 required, on average, so much 20mm fire (allowing for the fact that only 2-5% of the shots fired hit the target) that the effective range of their fighter guns was very short, because they had to get very close to ensure enough hits. 

Well, I don't know about that. Here the shooting accuracies the Luftwaffe considered to be realistic against heavy bombers under combat conditions: 

d (m) - Ph MG151/20 - Ph MK103 - Ph MK214 
500 - 9.1% - 10.0% - 10.5% 
1000 - 3.3% - 3.8% - 3.8% 
1500 - 0.8% - 1.3% - 1.5% 

That's why they switched to the low-velocity 30mm MK 108 late in the war. On paper, the poor trajectory and long flight time of the 30mm shells gave them a short range, but in practice their devastating effect on the target meant that few hits were needed, so hit probability could be low (i.e. they could fire at long range). As a matter of interest, the Luftwaffe found the low-velocity MK 108 more effective than the high-velocity 30mm MK 103 at any given range, because for the same weight a plane could carry two MK 108 (throwing out 1,200 rounds per minute) for every MK 103 (around 400 rpm), and putting three times as many shells in the air gave a better hit probability than with the high-velocity gun.

With regard to actual combat ranges, the MK108 of the Bf 109G-6 was sighted for 400 m distance. That means that they meant to use it at least out to that distance, and probably a bit beyond. From the trajectory, the weapon could be sensibly sighted out to 500 m, but drop after that is sharp.

The gondola MG151/20 were sighted for 500 m, but that might have been to make them match the MK108. Convergence range was 300 m, so that meant that they expected to press attacks at least to that range, and probably a bit closer.

For the Fw 190A-8/R1 with 6 x MG151/20, the cannon were sighted for 550 m, but convergence was at 900 m for the wing root cannon and at 800 m for the gondola cannon! I'd say they meant to use all that firepower to kill from farther out, and perhaps to create a large pattern. (I believe the Sturmbock with shorter effective range and the armour to survive the counter-fire was more successful 

The standard Fw 190A-8 with 4 x MG151/20 had the cannon sighted for 550 m, too, but convergence range was 600 m for the wing root cannon and 400 m for the outboard cannon, so this indicates a bit shorter ranges.

For the MK108-armed Me 262, standard tactics were to open fire at 600 m range, but as that was during a rapid overtaking attack, it might be that this was to get some tracers into the air just before getting into effective range in order to exploit the short engagement time most efficiently 

So Luftwaffe tactics probably expected the following normal engagement ranges:

Bf 109G-4/U4 with gondolas: 450 - 200 m
Fw 190A-8 with 4 cannon: 600 - 300 m
Fw 190A-8 with 6 cannon: 800 - 400 m
Me 262: 500 - 200 m (short range determined by speed 

Of course, the Sturmböcke closed to even shorter range, it was not the attacker's weapon that determined minimum range but the defenders' firepower 

But the above is what one can sensibly conclude from the information on gunnery setups. 

It would be interesting to see the convergence ranges for wing-mounted MK108s, but I haven't seen those anywhere yet.

Regards,

Henning (HoHun)


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## Soren (Jul 11, 2008)

Excellent posts HoHun..


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## Tony Williams (Jul 11, 2008)

A couple of points:

German sources obviously differ about the percentage of shots fired which hit the targets: I have seen two figures quoted, of 2% and 5%.

The reason I mentioned that pilots had to close to short range to ensure a kill with 20mm guns against a B-17 is that the Luftwaffe calculated that at long range, such a small percentage of hits would be scored that the fighters would run out of ammo before scoring the necessary 20+ hits to score a kill. The 30mm shells were so much more destructive that they solved this problem.

The second point is this: just because fighter guns were harmonised for a particular range doesn't mean that this was the expected kill range. Remember that the planes would have been closing with their targets quite quickly, so if they started firing at 600m they might finish firing - and achieve a kill - at 300m or less. The idea was to keep firing as they closed until they downed the target or ran out of ammo.

In the BoB, the RAF fighters initially had their guns harmonised for 400 yards, but the thinking was that this pattern would work well against bombers at short range, because it would spread their fire across the fuselage and both engines. So the fighters were expected to engage the enemy at 400 reducing to 200 yards, with the most effective fire being at the shorter range. They subsequently discovered, of course, that a better choice was 250 yards reducing to whatever it took, and adjusted the harmonisation accordingly.


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## HoHun (Jul 11, 2008)

Hi Tony,

>German sources obviously differ about the percentage of shots fired which hit the targets: I have seen two figures quoted, of 2% and 5%.

Well, obviously the above figures are quite accurate and specific to the respective weapon and range, while "2%" or "5%" look much like arbitary assumptions.

The "figure of merit" calculation for anti-bomber weapons (Bad Eilsen, 8.2.1945) arbitrarily assumes an average 5% hit rate regardless of the weapon type for the purpose of estimating the ammunition supply, but obviously for that purpose you have to make a conservative estimate because you have to allow for that half of the pilot with a below-average actual hit rate - this half of your pilots would run out of ammunition if you'd give them exactly the amount of ammunition required for a kill by an average pilot.

>The reason I mentioned that pilots had to close to short range to ensure a kill with 20mm guns against a B-17 is that the Luftwaffe calculated that at long range, such a small percentage of hits would be scored that the fighters would run out of ammo before scoring the necessary 20+ hits to score a kill. 

Well, the more specific report the above hit percentages are from suggests an average hit rate of 9.1 % for the MG 151/20 at 500 m, making the 95 % kill probability possible with the expenditure of 275 rounds.

For the MK 108, the hit percentage at the same range was estimated as 8.3 %, achieving the 95 % kill probability with just 48 rounds.

So it's not like 500 m were considered a prohibitively long range, and it's not like the 20-mm-cannon-armed fighters would routinely run out of ammunition. It's just that one MK 108 with 48 rounds was a lighter package than two MG 151/20 with 275 rounds, for the same end result. Obviously, that made the MK 108 the better choice.

>The second point is this: just because fighter guns were harmonised for a particular range doesn't mean that this was the expected kill range. 

It certainly means that it was the expected effective range. And effective range is the range at which a kill or serious damage will result from hits. Closer is always better, and firing a Luger from the co-pilot's seat obviously will be terminally effective. That doesn't mean that longer-range fire could be considered ineffective through a reverse conclusion ...

Regards,

Henning (HoHun)


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## Tony Williams (Jul 11, 2008)

HoHun said:


> Hi Tony,
> 
> >German sources obviously differ about the percentage of shots fired which hit the targets: I have seen two figures quoted, of 2% and 5%.
> 
> Well, obviously the above figures are quite accurate and specific to the respective weapon and range, while "2%" or "5%" look much like arbitary assumptions.


I have to say that the precision of the figures you quote makes me suspicious. How could they know what hit rates were achieved at different ranges in the chaos of combat? The only possible sources for this information - the aircrew involved - would have been far too busy to make a careful note of this even if they knew, particularly since estimation of the ranges in air-to-air combat was notoriously inaccurate.



> >The second point is this: just because fighter guns were harmonised for a particular range doesn't mean that this was the expected kill range.
> 
> It certainly means that it was the expected effective range. And effective range is the range at which a kill or serious damage will result from hits.



I would say that it represents the *maximum *range at which hits might be expected to be scored. As we know, a B-17 required many 20mm hits to be brought down, so it could take a long time to acccumulate those hits at long range.


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## Kurfürst (Jul 11, 2008)

Tony Williams said:


> I have to say that the precision of the figures you quote makes me suspicious. How could they know what hit rates were achieved at different ranges in the chaos of combat? The only possible sources for this information - the aircrew involved - would have been far too busy to make a careful note of this even if they knew, particularly since estimation of the ranges in air-to-air combat was notoriously inaccurate.



Gun camera films. The ones I have seen from the Luftwaffe specifically mention the range at the time of opening fire, and also what the pilot actually though the range was (typically they thought they were much closer than they really were).

Range can be estimated rather easily using the wingspan of the enemy aircraft as reference from these films, and probably there was some kind of way telling how many rounds were fired - if by no other means, tracers would tell when the weapon started/finished firing, total time was known from how many frames were shot; hits could be observed visually.


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## Tony Williams (Jul 11, 2008)

Kurfürst said:


> Gun camera films. The ones I have seen from the Luftwaffe specifically mention the range at the time of opening fire, and also what the pilot actually though the range was (typically they thought they were much closer than they really were).
> 
> Range can be estimated rather easily using the wingspan of the enemy aircraft as reference from these films, and probably there was some kind of way telling how many rounds were fired - if by no other means, tracers would tell when the weapon started/finished firing, total time was known from how many frames were shot; hits could be observed visually.



Fair point - I should have thought of that!  

However, hits could not necessarily be observed, since (especially against bombers) cannon shells were fitted with delayed action fuzes intended to explode within the plane, not on the surface.


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## HoHun (Jul 11, 2008)

Hi Tony,

>The only possible sources for this information - the aircrew involved - would have been far too busy to make a careful note of this even if they knew, particularly since estimation of the ranges in air-to-air combat was notoriously inaccurate.

As Kurfürst already pointed out, the report was in fact based on gun camera films from combat situations, but additionally the results from non-combat trials were taken into account, too. (The report only mentions this in passing.) 

I think the air forces were so well aware of the range estimation problem you pointed out because they were routinely comparing these films with the corresponding combat reports by the pilots.

>I would say that it represents the *maximum *range at which hits might be expected to be scored. As we know, a B-17 required many 20mm hits to be brought down, so it could take a long time to acccumulate those hits at long range.

Hm, since the lethality of your fire depends on the concentration of hits, you would always want to have the convergence distance closely match with the typical range at which kills are achieved.

In fact, the "maximum" range (whatever this means  is always beyond convergence distance because the geometry of the diverging trajectories does not lead to a sharp drop-off of lethality, but to a gradual one. (Making it more difficult to define the "maximum" effective range.)

Here is the complete hit percentage table as used in the detailed report:


```
Range  - 500 m - 1 km - 1.5 km
MG 151 -  9.1% - 3.0% - 0.8%
MG 213 - 10.0% - 3.8% - 1.2%
MK 108 -  8.3% - 3.3% - 0.9%
MK 103 - 10.0% - 3.8% - 1.3%
MK 412 - 10.0% - 3.8% - 1.3%
MK 214 - 10.5% - 4.2% - 1.5%
```

Note that the ca. 10% hit percentage at 500 m goes well with the 5% assumption from the "Bad Eilsen" overview since you'd have to allow a greater ammunition supply to make sure to have enough in a below-average attack.

The number of hits required per weapon according to the report:


```
Pk     - 50% - 95%
MG 151 - 18 -  25
MG 213 - 18 -  25
MK 108 -  4 -   7.3
MK 103 -  4 -   7.3
MK 412 -  1 -   2.8
MK 214 -  1 -   2.8
```

Note that this is slightly more than the figures often quoted in popular books. I guess this might be because the "Bad Eilsen" overview does not quote the Probability of Kill (besides using slightly different figure of 20 hits for the MG 151/20 and 5 for the MK 108) achieved with the stated number of hits. From the direct comparison, I'd say it is only 50%, making the popular "5 hits for a kill" statement a bit suspect.

The weight of the weapon including mounting and ammunition required for a 95% Probability of Kill (Pk) from 500 m:


```
Pk     -    kg - firing time
MG 151 - 119.7 -  17 s
MG 213 - 194.3 -  11 s
MK 108 - 140.4 -   5 s
MK 103 - 267.7 -   6 s
```

The report originally was provided by Karl over at alt.games.warbirds. Unfortunately, I have only two of the pages, so some details are missing 

Regards,

Henning (HoHun)


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## Tony Williams (Jul 11, 2008)

Thanks for posting that information, Henning, although I am still a bit sceptical of the precision of the numbers. Having had some peripheral involvement with statistics in my career, I do know that impressively precise numbers sometimes obscure lots of rough estimation behind the calculation....In particular, I doubt that the MK 214 fired more than handful of shots in combat.



HoHun said:


> Hm, since the lethality of your fire depends on the concentration of hits, you would always want to have the convergence distance closely match with the typical range at which kills are achieved.
> 
> In fact, the "maximum" range (whatever this means  is always beyond convergence distance because the geometry of the diverging trajectories does not lead to a sharp drop-off of lethality, but to a gradual one. (Making it more difficult to define the "maximum" effective range.)



An interesting subject which I'm sure we could debate for hours! Having read a lot of contemporary material in the National Archives on the RAF convergence patterns in the BoB, I don't agree that the convergence pattern was usually selected to match "the typical range at which kills were achieved"; I believe that it was normally chosen to match the maximum range at which a kill became feasible.

Don't forget that there were several factors affecting the distribution of fire from fighters: the convergence pattern was only one of them. Others were the basic dispersion factors for the guns in their mountings (the .303 Browning spread its shots over a 1 metre diameter circle at 100 metres - so at 500 metres its fire would be spread over a 5 metre circle, or about 20 square metres); another factor was "aim wander" - no pilot could keep his plane pointed at exactly the same point during a burst of fire, there was always some spread due to aircraft movement. Shot dispersion and aircraft movement would, in combination, mean that the fire from aircraft would in any case be much more widely spread at long range than at short range - so it made sense to converge the gun aiming points at long range to try to compensate for that. 

An interceptor could be expected to be closing on its target quite quickly when attacking. A B-17 cruised at less than 200 mph while an interceptor could be expected to be attacking at at least double that speed, maybe more. A closing speed of 200 mph is almost 100 metres per second; so if the interceptor opened fire at 500 metres range and fired a 3-second burst, he would be at only 200 metres range when he finished. And that's in a tail attack: head on, the closing speed could be over 500 mph or 250 metres per second; if he started firing at 500 metres he would scarcely have any time to shoot before having to pull up to avoid a collision. So just because convergence distances were set for a particular range, it did not mean that most kills were achieved at that range - it merely represented the starting point of the attack.

I think that the most realistic convergence patterns were those chosen by those US pilots who set pairs of .50 cal guns to converge at different distances. The outermost pair was set to long range, the next closer and the inboard pair to close range. This gave a reasonable concentration of fire at all feasible combat ranges.


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## HoHun (Jul 12, 2008)

Hi Tony,

>Thanks for posting that information, Henning, although I am still a bit sceptical of the precision of the numbers. 

The precision of course is only valid in the exact context. What's important is the magnitude, and that is 10% at 500 m for all of the weapons considered.

>Having read a lot of contemporary material in the National Archives on the RAF convergence patterns in the BoB, I don't agree that the convergence pattern was usually selected to match "the typical range at which kills were achieved"; I believe that it was normally chosen to match the maximum range at which a kill became feasible.

I think this is a misconception that results from the lack of definition for "maximum effective range", which in this case in implied in the word "feasible". There is nothing magic about the convergence distance that causes bullets to become ineffective once they fly past it. 

>Don't forget that there were several factors affecting the distribution of fire from fighters: the convergence pattern was only one of them. Others were the basic dispersion factors for the guns in their mountings (the .303 Browning spread its shots over a 1 metre diameter circle at 100 metres - so at 500 metres its fire would be spread over a 5 metre circle, or about 20 square metres); 

Well, if you look at the trajectory of the MK 108 at 500 m, the pattern centrepoint will be just 80 cm below the crosshairs, and all hits will strike within a 75 cm circle. Despite (or in actually, because of!) its low muzzle velocity, it's a precision weapon. You really shouldn't make any conclusions from 7.7 mm machine guns as mounted in RAF fighters, they're just too different.

>Shot dispersion and aircraft movement would, in combination, mean that the fire from aircraft would in any case be much more widely spread at long range than at short range - so it made sense to converge the gun aiming points at long range to try to compensate for that. 

Eglin Air Foce Base test pilot Don Lopez in "Fighter Pilot's Heaven" notes that in his gun-camera recorded tracking tests, the optimum tracking result he achieved was an accuracy of 0.5 mils. At 500 m, that's an error of just 25 cm. He also mentions that a tracking accuracy of 6 mils was considered acceptable by the US, which is a 3 m error at the distance we are looking at. Considering the size of a four-engined bomber, that is nowhere near enough to make it "non-feasible" to hit it even if its far beyond convergence distance.

The adjustment of the convergence distance you are speaking of would not be chosen to give the best destructive effect at the worst possible firing range, but rather the best destructive effect at the most useful firing range.

You have to look for the integral of Pk over time in the overtaking attack you describe - that is the figure the convergence distance will be chosen to maximize. It certainly would be an operational research screw-up to put it at some extreme limit connected with hit chances appproximating zero. (However that limit might have been defined.)

Regards,

Henning (HoHun)


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## pete_madi (Jul 12, 2008)

tony if you check youtube they have some good gun camera footage mainly german a/c


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## drgondog (Jul 12, 2008)

Another factor not considered in the above 'bore sight' discussions is that nose mounted weapons on a 109/P-38, etc should be more reliably close to ground bore sight results than wing mounted weapons.

The inboard weapons for an Fw 190, situated in the root, should be next and all wing mounted weapons outboard of main gear are subject to the wing bending under aero loads - which in my opinion makes them the least reliable platform relative to gunsight/weapon convergence.


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## Tony Williams (Jul 13, 2008)

Henning, there is a big difference between tests on the ground, or under practice conditions, and what actually happened in combat.

I refuse to believe that an MK 108, fired in combat from a plane which was constantly having its course corrected and probably bucking in the slipstream of the bomber formation it was attacking, could possibly group its fire within 75 cm at 500 m range, or anything like it.


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## HoHun (Jul 13, 2008)

Hi Tony,

>Henning, there is a big difference between tests on the ground, or under practice conditions, and what actually happened in combat.

Well, the Baade report actually notes that the dispersion measured in flight tests typically is smaller than the dispersion measured in ground tests. If anything, my numbers are conservative as a result.

>I refuse to believe that an MK 108, fired in combat from a plane which was constantly having its course corrected and probably bucking in the slipstream of the bomber formation it was attacking, could possibly group its fire within 75 cm at 500 m range, or anything like it.

Re-reading the report, it must actually be 1.5 m at 500 m range - they use the diameter symbol, but in the text explain that the figures are for the 100% hit radius.

Still, have you any figures for cannon dispersion at all? The MK 108 in a nose mounting certainly should be near the low end of the range due to its low muzzle velocity, its operating principle, and due to the stiff mount. The MG 151/20 and the MK 103 due to their higher muzzle velocity have a greater dispersion, the low-velocity MG FF has even less.

Bomber slipstream ... sure, it's a negative effect, but that's why the Luftwaffe pilots were taught to avoid attacking through the slipstream. This has nothing to do with the weapon at all.

It's obvious that if the pilot would make an optimum gunnery run with 0.5 mil error, shooting with a weapon that has 1.5 mil dispersion, a bomber-sized target would be hit with 100 % probability at 500 m. The Luftwaffe expects a hit probability of around 10 % in combat, and there is no technical reason that shouldn't be possible as the main source of error is the pilot, not the cannon or the airframe.

Regards,

Henning (HoHun)


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## HoHun (Jul 19, 2008)

Hi again,

>The dispersion strongly varies between different mountings, and for some reason the B-17 tail turret is the least accurate mounting (except for the even more inaccurate hand-held tail guns).

Reading the manual for the German FDL-B 131 remote control turret as used for the B turret in He 177, armed with a 13 mm MG 131, it seems that the mechanical aiming error of that turret is expected to be 10 mil judging from the calibration procedure.

Accordingly, the dispersion of the weapon would be somewhat above that 10 mil figure, so it would probably be roughly in the same order of magnitude as for the US turrets (Sperry upper turret - 11.7 mil, Bendix chin turret - 12.5 mil). Depending on the rigidity of the mount, I'd guess dispersion might even be slightly larger than these values.

(Calibration requires four bursts of seven shots each to be fired after pointing the turret at the aim point from the left, right, bottom and top respectively. The centre of each group of hits should be with 50 cm of the aim point at 100 m range.)

Regards,

Henning (HoHun)


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## seesul (Jul 23, 2008)

OK, and back to the B-17 with the 20mm cannon instalation.
I got scanned the picture and it is in jpg file this time so I´ve attached it again.
The text written in the book says:
Opposite centre left: a 20mm cannon in the tail position of the 97th BG´s B-17G 42-38090, one of at least 5 such installations made by this group in the spring of 1944. The sight was a ring-and-bead on the end of the long rod attached to the gun barrel. As M/Sgt Morley Russell, who worked on this installation, commented,´The sight sure was a real Rube Goldberg type, but it worked well.´ (Morley L. Russelll)
Opposite bottom: B-17G 42-38090, on a mission in July, displays the 20 mm cannon installation. Because of the limited movement of the gun it had a poor field of fire, but its purpose was to counter the enemy fighters that fired rockets from outside the range of the normal .50 calibre armament. B-17s with these tail cannon were flown at the rear of formations, and the weapon was considered more of psychological that destructive value (Steve Birsdall).

Roger Freemann/David Osborne-The B-17 Flying Fortress story, page # 44-45


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## seesul (Jul 23, 2008)

But I have a question- on the end of Hispano barrell there´s something like one more barrel, or at least a bush...does anyone know what is it for?
Or is there .50 mounted together with the M2 Hispano???


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## seesul (Jul 24, 2008)

Anyone here from Michigan state that could help me with research?
Morley L. Russel, 90 years old, lives in Shelby Township, Michigan, according to white pages.
Dunno if is him, who made this 20mm cannon installation, but the name is the same. Could be nice and interesting to find him, ask him few questions and show him the pics I have...
Thanks in advance!


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## seesul (Oct 6, 2009)

Here´s the actual list of the Forts with the tail mounted M2 Hispano I´ve found by now:

42-31885 (99th BG, later 2nd BG) - picture from the crash site
42-32046 (483rd BG, later 99th BG)- The B-17 Flying Fortress Story by Roger Freeman, note on page 174
42-38090 (97th BG)- The B-17 Flying Fortress Story by Roger Freeman, pic of the tail on page 44, note on page 45
42-97490 (2nd BG)- Konečná zastavka Slovensko (Final Destination Slovakia) by Peter Kassak, pic of the wreckage,page 84
42-38087 (2nd BG)- mentioned in the 2nd BG database
42-31590 (2nd BG)- mentioned by Mr. Steve Birdsall
42-97438 (2nd BG)- mentioned by Mr. Steve Birdsall


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## B-17engineer (Oct 6, 2009)

Nice job with the info Roman! Looks interesting! The chine turret with six guns looks lethal


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## seesul (Oct 6, 2009)

B-17engineer said:


> Nice job with the info Roman! Looks interesting! The chine turret with six guns looks lethal



But this chin turret must have been pretty hungry for shells...I´d like to see its feeding system...


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## vikingBerserker (Oct 6, 2009)

Wow, that's pretty sweet. Never even heard of a 6 gunned turret before.


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## seesul (Feb 18, 2011)

The picture on the first page of this thread shows the wreckage of B-17G, 42-31885 that crashed close to my born town.
We weren´t sure where the 20 mm cannon was but the most probably position was in tail. Now, after reading the book 'Mighty By Sacrifice' on page 92-94, where Bob Donahue, a tail gunner desribes his experince from July 9, 1944 during the raid on Ploesti, we know more. He says there that on this mission he manned a 20 mm cannon. From the database of 2nd BG was easy to find out that he flew 42-31885 that day. So 20 mm cannon instalation on this machine in tail confirmed from another source.


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## drgondog (Feb 18, 2011)

One of the bomb groups in 8th AF experimented with a 20mm nose mount but took it out because the nose had no load paths to carry the stress of the recoil.


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## mikewint (Feb 18, 2011)

Change of topic for a bit, but I was surprised that the B-29 had a rear mounted 20mm plus two .50 cal machine guns. Then after WWII the 20mm was removed leaving only the twin .50 cal in the rear. Now I see that the AF was trying the 20mm out in the B-17 as well


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## seesul (Feb 20, 2011)

drgondog said:


> One of the bomb groups in 8th AF experimented with a 20mm nose mount but took it out because the nose had no load paths to carry the stress of the recoil.


Yep, also 15th, picture posted on the previous page.


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## Multimetal (Feb 22, 2011)

I know that a few Luftwaffe bombers carried 20mm guns in defensive positions also, specifically the FW 200 had either an MG/FF or later a MG 151/20 mounted at the front of the gondola. I think with the FW 200 the intention was more for suppressing AA fire from ships but it was surely used as a defense from fighters too. Also the Japanese Betty carried a 20mm in the tail position. I wonder how they overcame the traversing and recoil problems that kept the 20mm from being effective in the B-17? I'd never heard of the 20mm installations in B-17's before, that's really interesting! Learn something new every day I suppose...


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## Shortround6 (Feb 23, 2011)

Multimetal said:


> Also the Japanese Betty carried a 20mm in the tail position. I wonder how they overcame the traversing and recoil problems that kept the 20mm from being effective in the B-17?



A different 20mm cannon that was shorter, lighter and used a much less powerful cartridge.


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## drgondog (Feb 23, 2011)

Multimetal said:


> Also the Japanese Betty carried a 20mm in the tail position. I wonder how they overcame the traversing and recoil problems that kept the 20mm from being effective in the B-17? I'd never heard of the 20mm installations in B-17's before, that's really interesting! Learn something new every day I suppose...



The key difference is that the German and Japanese bombers were designed and structured for the recoil. The B-17 was designed fo a .30 caliber in a ball socket and nothing significant was done until the YB-40, then the B-17G, to take recoil in the nose compartment.

All the mods to 'cheek' and nose mounts in the B-17E were done in the field. The B-17F was first to have a factory mount in front for .50 caliber in either cheek, but the mount for the bombadier was also done in the field - and that was the basis for the 20mm 'attempt' - which beat hell out of the nose and deemed dangerous.

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## Multimetal (Feb 23, 2011)

I've also seen pictures of a field-modded B-17 that had a set of twin .50's cut into the plexiglass nose. It seems like the recoil and vibration would be close to that of a single 20mm so maybe it wasn't deemed effective either.


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## drgondog (Feb 23, 2011)

Multimetal said:


> I've also seen pictures of a field-modded B-17 that had a set of twin .50's cut into the plexiglass nose. It seems like the recoil and vibration would be close to that of a single 20mm so maybe it wasn't deemed effective either.



Most B-17F's in the 8th AF had that twin mount as a field mod before the chin mount came with all factory B-17G's..


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## seesul (May 12, 2020)

After several years back to this topic. 20 mm cannon in tail story at 2nd BG is getting more and more interesting. So far we know that on Aug. 29, 1944 at least 3 Forts were equipped by this experimental gun. S/N 42-38096 (Big Time, 20 mm shells found at the crash site), 42-31473 (My Baby, 20 mm shells found at the crash site) and 42-31885 (20 mm shell found at the crash site plus pics from the crash site showing this gun).
But today I spent some time on the Narrative Mission Report and found something interesting again. In the point 6 of the Daily Operations Report there's written:
0.50 API&T - rounds carried 235.400, rounds expended 21.730, rounds lost on missing & destroyed aircraft 57.240.
And now the Holy Grail - 20 mm - rounds carried 1200, rounds expended 100, rounds lost on missing & destroyed aircraft 600...
So, 2nd BG sent out 28 machines, some of them carried 20 mm cannon in tail and half of them was a among those lost 9 machines (1200 20 mm rounds carried and 600 20 mm rounds lost with destroyed machines). So it means that 2nd BG itself had a significant number of the machines with this "toy".
How many? A good question - Hispano M2 20 mm cannon had a 60 rounds magazine. Should each of those 9 Forts that were shot down have one magazine only, it would make 540 rounds lost with the machines. Report says 600 rounds lost so it's clear that those machines had to have at least 2 magazines. And I also think, that because of weight, that was it. 2 magazines make 120 rounds per machine. B-17s were tail-heavy (F version for sure more than G) and I doubt that they would overload the rear part more than necessary needed. 600 rounds lost with the machines would then mean 5 machines lost with this gun and also other 5 machines with this gun that made it back. So on my opinion 10 (!) B-17Gs from the 2nd BG were equipped by 20 mm cannon in tail on that day. I may be wrong but my interrogation goes on.

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## GrauGeist (May 12, 2020)

Fantastic research, Roman!

I knew that the 20mm was used on several B-17s (in both ETO and PTO), but I had no idea there were so many!

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## seesul (May 12, 2020)

Thank you David. Later today I´ll post a list of the 10 machines from 15th USAAF with this modification I have so far.


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## GrauGeist (May 13, 2020)

Looking foreward to it


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## seesul (May 13, 2020)

I have to correct my calculations above. I friend of mine told me that 20 mm cannon fitted in B-17 tail didn´t use the drum magazine but the belt feeding system. So there's still is a question have many machines from the 2nd BG were equipped by cannon on August 29, 1944 but taking into consideration that at least 3 of the machines that crashed here (42-31473, 42-31885 and 42-38096) had cannon and the fact that this group took of with 1200 20 mm rounds and that 600 rounds were lost with the machines it would mean that at least 3 other machines with 20 mm cannon got back to the base on that day.

Anyhow, here is the list of the machines with 20 mm cannon in tail that I have so far:

41-9023 (E version, 91 BG) - B-17 IN ACTION BY STEVE BIRDSALL SQUADRON/SIGNAL PUBLICATIONS with a note that the project was abandoded because of popping rivets during recoil
42-30267 (F version, 2 BG + 97 BG) - source 5TH BOMB WING HISTORY OF AIRCRAFT ASSIGNED - DICK DRAIN
42-31473 (2nd BG) - 20 mm shells found at the crash site
42-31590 (2nd BG) - source Mr. Steve Birdsall
42-31885 (99th BG, later 2nd BG) - picture of the cannon from the crash site
42-32046 (483rd BG, later 99th BG)- source The B-17 Flying Fortress Story by Roger Freeman, note on page 174
42-38087 (2nd BG)- mentioned in the 2nd BG database
42-38090 (97th BG)- The B-17 Flying Fortress Story by Roger Freeman, pic of the tail on page 44, note on page 45
42-38096 (2nd BG) - 20 mm shells found at the crash site
42-97438 (2nd BG)- source Mr. Steve Birdsall 
42-97490 (2nd BG)- source Konečná zastavka Slovensko (Final Destination Slovakia) by Peter Kassak, pic of the cannon,page 84

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## GrauGeist (May 14, 2020)

Great info, Roman!

In photo #6, is that a .50 MG mounted alongside the 20mm canon?

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## seesul (May 15, 2020)

Yes David, also on the pic No. 3

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## GrauGeist (May 15, 2020)

wow...I missed the details in pic. #3 (I'm on a smart phone), but when I expanded the image, I also noticed that it's an earlier B-17 (pre-G model) and it appears that the gunner's position has custom armor around the gunner's canopy.


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## Dimlee (May 23, 2020)

It reminded me one episode from a good book 

_The voice from the tower was our salvation: “Return to your parking spots, all daylight bombing has been canceled, by order of General Hoyt S. Vandenberg, Chief of Staff USAF.”
Not long afterward the man himself visited B-29 units in search of better ways to deal with the MiG menace. I have a color slide of the four-star general surrounded by B-29 officers while listening to an enlisted tail gunner...
One of the first things General Vandenberg said was, “I want to talk to a tail gunner.” One gunner eased forward. Vandenberg asked him, point blank, “Would you guys like the 20mm reinstalled in the tail?” They all replied in the affirmative, but I don’t think it was ever done, probably because the trajectory of the shells fired from the 20mm cannon was completely different from that of the bullets from the 0.50-inch machine guns, which made aiming in combat even more difficult._

(J. McGill, Earl.. Black Tuesday Over Namsi: B-29s vs MIGs—The Forgotten Air Battle of the Korean War, 23 October 1951 . Helion & Company Ltd.. Kindle Edition. )

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## seesul (Sep 7, 2020)

Here a picture that was taken in the museum in Slavicin, Czech Republic a week ago.

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## Acheron (Sep 7, 2020)

Did you reach any consensus on whether or not a 20-mm gun was worth it? I would assume the inlined Me-109 to be not very tough, and to my knowledge, at the altitudes the B-17s flew, the FW-190s performance went down hard, no?

And what were the differences in hit probability, given the lower RoF?


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## seesul (Sep 7, 2020)

Acheron said:


> Did you reach any consensus on whether or not a 20-mm gun was worth it? I would assume the inlined Me-109 to be not very tough, and to my knowledge, at the altitudes the B-17s flew, the FW-190s performance went down hard, no?
> 
> And what were the differences in hit probability, given the lower RoF?



As written in one of the books this gun had more psychological effect for the bombers crews than the destructive effect. Lower rate of fire, limited field of fire, hits accurancy and a different trajectory. Should it be an effective improvement more machines would be modified. So far I have found 1 Fort with this gun at 8 USAAF and this project was abandoned and 10 Forts at 15 USAAF. My estimation of the number of the modified machines at 15th is about 20 pcs.


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## Shortround6 (Sep 7, 2020)

Dimlee said:


> but I don’t think it was ever done, probably because the trajectory of the shells fired from the 20mm cannon was completely different from that of the bullets from the 0.50-inch machine guns, which made aiming in combat even more difficult









out to 500 yds the trajectories were never more than a few inches apart. Beyond that range trajectory and time of flight did start to differ more. The Hispano and the .50 Browning were probably one of the better match ups as far as trajectory and time of flight go.

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## seesul (Sep 8, 2020)

Shortround6 said:


> View attachment 594192
> 
> 
> out to 500 yds the trajectories were never more than a few inches apart. Beyond that range trajectory and time of flight did start to differ more. The Hispano and the .50 Browning were probably one of the better match ups as far as trajectory and time of flight go.



Thank you for this table and comparison. The main purpose of the 20 mm cannon installation was to keep the German fighters away in a longer distance yet before they were able to open an effective fire.


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