B-17G with a 20 mm cannon onboard!

[Soren]

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.

 

[HoHun]

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)

 

[Glider]

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.

 

[seesul]

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

 

[Soren]

Gotta second what HoHun says..

 

[Tony Williams]

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.

 

[seesul]

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...

 

[Bigxiko]

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

 

[Flyboy2]

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

 

[Erich]

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.......

 

[seesul]

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..

 

[seesul]

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.......

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...

 

[Erich]

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 ?

 

[seesul]

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...

 

[Glider]

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

 

[Micdrow]

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

 

[HoHun]

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)

 

[Soren]

Excellent posts HoHun..

 

[Tony Williams]

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.

 

[HoHun]

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)