20/20 Hindsight - different armament?

[delcyros]

...taken an example from Lunatic from an earlier post, regarding recoil forces on mid wing mounted guns, the A1D Skyraider suffered very worsely if one of her two 20mm guns jammed, the recoil always was to strong to be trimmed out.
This eventually lead to the four 20 mm gunned Skyraider (Recoil-weight ratio:0.076), not because they wanted more firepower but because if one gun jammed and the other mate was out due to recoil issues they had one pair left.
This shouls be true for the P-47 as well.
The Spitfire IXc is listed with four 20mm cannons (recoil-weight ratio: 0.16) but due to recoil and performance issues almost always flew with only two 20mm guns. The RCAF No. 421 squad flying the four gunned version almost entirely flew ground combat sorties with them.
Factoring recoil, weight and jamming rates, the six 20mm are truly no improvement over eight 0.50cal guns. You gain destructive power but You do loose accuracy, platform stability, performance. Personally I would upgun the P-47 with six to eight 0.60cal guns. They do not have such a strong recoil compared to 20mm, significant more projectile weight than a .50cal, a decent muzzle velocity + a comparable rof (which is improvable had they concentrated on this). The higher sectional density gives them extreme accuracy due to an even flatter trajectory and shorter time to distance figures, a better punch, a longer effective firing range and I do suspect a better armour penetrating capability as well (at least for low impact obliquities). So You get improvements in both main aspects with this gunnery layout for our P-47: Improved hit probability and better punch instead of trading the former for the latter. With only six 0.60cal and 350 rpg, the planes weight +their handling performance remains almost the same, the recoil-weight ratio is 0.059, well in within excellent figures for fighter A/C.

 

[Jank]

"Factoring recoil, weight and jamming rates, the six 20mm are truly no improvement over eight 0.50cal guns."

Earth to Delcyros, we've already been over this. The weight of six 20's with 175 rounds per gun is about the same as eight .50's at full ammunition load. Thus, weight is not an issue to be factored in.

As to recoil, if the Hurricane handled 4 x 20mm's just fine, then a P-47 would handle 6 just fine and with less recoil force per pound of aircraft. You still have produced nothing convincing me or anyone else that the 4 x 20mm armed hurricanes "suffered" in the air to air role or were relegated to the air to ground role because of that. The Hurricane, to be brief and simple, sucked, and was no longer useful as an air superiority fighter. That is why the 4 x 20mm armed Hurricane as well as the others, were generally withdrawn from air to air duty. It did, however, perform well in the close support ground attack role where speed and climb were not determinative of success.

The issue of jamming favors a six vs. a four gun installation as well. Again, I am only making the point that a six 20mm armed Thunderbolt, late in the war, after they had largely been shifted to ground attack and close ait support, would have benefitted from such an installation.

 

[delcyros]

Earth to Delcyros, we've already been over this. The weight of six 20's with 175 rounds per gun is about the same as eight .50's at full ammunition load. Thus, weight is not an issue to be factored in.

The key word was "improvement". Using six 20mm will make the airframe heavier (compensated to some degree by the reduced ammo and the use of MK V instead of MK II, as You suggest. However, ammo boxes, belt weight and gunmount weights are not taken into consideration by Your calculation and they do weight a lot, also). Cutting the ammo to 175 rpg equals to only 14 s. of firing volume instead of 34 s. for M2 with 450 rpg and 29 s. for cal.60 with 350 rpg. Something to worry about for a ground attacker.
The recoil length of the shorter barelled MK V also points to a higher peak recoil despite a somehow lower brutto recoil in comparison to the MK II. Special gunmounts may reduce this effect, as Tony suggested, altough I am sure they don´t come with zero weight.
Regarding to the Hurricane, I found many quotes regarding the repeated redesign of the wing structure to handle recoil weight´s of the 20mm and I found no pilot quote stating that the plane was "just fine" with four 20mm. All comments regarding the excellent gunstability to proof the Hurricane was fine relate either to the 0.303 cal equipped version if You trace them back (numerous accounts at BoB) or to the soviet modified, which shouldn´t surprise us much.
In the end I am confident that The P-47 with six 20mm is both, overgunned, and unnecessarily limited in purposes. This weaponlayout has more limiting factors than benefits. If You pretend on this, I do believe now that it may be possible to modify the plane in the way You suggested, but as I did earlier, I question the sense of such measures.

 

[Twitch]

Isn't this where it all began like 9 pages ago with most folks agreeing that the P-47 could handle 4 or 6 20 mms? I am mystified as to why anyone needs to ponder that one! I simply look at XP-72 armament variants and figure the Republic engineers had a better handle on it that we do. Whatever. Anyhow, right the P-47 did just fine the way it was and didn't "need" a brace of 20 mils.

Rüstätza were field conversions sets for armament mods in the field. Please note there were many custom "one off" conversions of Luftwaffe aircraft that were not mainstream depicted in the ubiquitous "stat" books. Rüstätza were "kits" to beef up armament on just about every type and sub-model of plane in combat. Luftwaffe armorers were very creative with these and they created alternate armament layouts that were not officially recognized in any way. Hence there aexist narratives describing these overkill gunships that pilots hoped would give them the ultimate advantage in firepower against the bomber stream. Simply because it seems improbably to us armchair crew chiefs 60+ years hence doesn't mean it is impossible.

 

[Jank]

"Factoring recoil, weight and jamming rates, the six 20mm are truly no improvement over eight 0.50cal guns."

The USN did testing, the methodology of which was unassailed, that indicated that round for round, a 20mm was 3 times as destructive at close range and 2.5 times as destructive at longer ranges than a .50.

For the air to ground role, it would indeed "truly" have been an improvement over an existing situation that was not a problem to begin with. No one ever said it was needed or necessary. This thread, like zillions of others, is just a "what if" inquiry.

"I found no pilot quote stating that the plane was "just fine" with four 20mm." That was funny because you wouldn't generally come across comments that things are "just fine" if that was the case. "Just fine" is simply not noteworthy. On the other hand, if things were "not fine," you would generally hear about it. Therefore if no evidence exists of comments that things were "just fine" and no evidence exists of comments that things were "not fine," and you had to draw a conclusion, the reasonable and logical inference would be that the absence of "not fine" comments means that it was "just fine."

"the A1D Skyraider suffered very worsely if one of her two 20mm guns jammed, the recoil always was to strong to be trimmed out."

I don't believe that for a second.

Sounds like you're arguing just for the sake of arguing which is "just fine" but I think this issue was resolved to everyone's satisfaction long ago.

 

[Squeelig]

I have read that it was a simple task to reduce a guns ROF which increased relaibility and reduced wear on the barrel. If the ROF was reduced to 600 rpm for the twenty milimeter, then the firing time would be increased from 14 seconds to 17.5 seconds which is very respectible. Less jams too. The total output would still be 3,600 ROF per minute which would be more than the 3,000 ROF per minute for a four gun armed 20 milimeter set up.

3,600 ROF per minute is equivalent to 9,000 RPF per minute in destructive power (2.5 X .50). The P-47 only had 6,000 ROF per minute for its fifty calibers. That's 50% more power at longer ranges.

At closer ranges, that would be like 10,800 ROF per minute of fifty calibers.

 

[delcyros]

The USN did testing, the methodology of which was unassailed, that indicated that round for round, a 20mm was 3 times as destructive at close range and 2.5 times as destructive at longer ranges than a .50.

For the air to ground role, it would indeed "truly" have been an improvement over an existing situation that was not a problem to begin with.


"the A1D Skyraider suffered very worsely if one of her two 20mm guns jammed, the recoil always was to strong to be trimmed out."

I don't believe that for a second.

For the latter part, ask Lunatic in his post 52
http://www.ww2aircraft.net/forum/aviation/why-us-only-nation-rely-50-a-2747-4.html
He is reputated for knowing what he said. So believe it or not, the Skyraider suffered from recoil issues.

The former part of your post, while interesting, tends to simplicate the matter. There is no overall agreed solution how to rate gunnery effectiveness, altough the comparsion in gunnery effectiveness
WORLD WAR 2 FIGHTER GUN EFFECTIVENESS
implys that the MK II is even four times as damage effective as was the .50cal.
However, against ground targets You need: armour penetration and a high volume of fire. Explosives have limited benefits most cases (unlike against airframes), except for soft targets like trucks etc. (altough the eight .50cal were sufficiant in destroying them and even steel made locomotives).
Regarding pure volume of fire, six 20mm (or .60cal) gives 75 rps while eight .50 cal give 104 rps, which is a significant disadvantage and don´t ease the probability of hitting something.
Regarding pure armour penetration the 20mm round is superior at high impact obliquities due to weight advantage but it shows no striking advantage over the 0.50 cal (and I suspect it is about the same or even less effective than 0.60cal) at low impact obliquities.
It´s ballistic coefficient of 0.555 allows for a quite significant drop in impact velocity for the MK V at 300 m (~630 m/s + planes vectoring speed), so I get via M79 APCLC an armour penetration sligtly less than 40 mm at best possible impact obliquities. In comparison to this the 0.50 cal 736 m/s @300 m + planes speed would give a penetration of slightly more than 41mm, the 0.60cal guns even slighly more than 44mm! -the higher the impact obliquity, the better the 20mm, of course-
Beside this, I would rate the .50 cal even better, because it may defeat thicker plates by repeated hits due to better accuracy and higher volume of fire.
What left from the big advantage of the 20mm? Explosive charge carried. But if You pretend on this advantage I would rather suggest to load more bombs for ground attacks...

Regarding the Hurricane issues, we already have overrun the repititive limit. I posted that the plane was redesigned in the VVS- surely not without good reason. I also posted that the wingdesign had to be modified repeatedly because of the problems associated with handling and firing the large 20mm guns. I am collecting pilot information and will post them later in case I get a sufficiant statistical base (I will also post in case it disproves my theory, anyway). But I suspect You rely on a position not justified to be an argument for or against.

 

[Jank]

Lunatic said, "With .50's, the recoil can be trimmed out so the mated gun can still be fired. Even on the AD Skyraider (a huge plane), if one 20mm jammed the mate on the opposite wing was useless."

To which you draw the obvious (to you) conclusion that, "He is reputated for knowing what he said. So believe it or not, the Skyraider suffered from recoil issues."

Like I said, "I don't believe that." That does not constitute evidence of anything. Just your faith in a "Lunatic"

"Regarding pure armour penetration the 20mm round is superior at high impact obliquities due to weight advantage but it shows no striking advantage over the 0.50 cal (and I suspect it is about the same or even less effective than 0.60cal) at low impact obliquities."

You have a vivid imagination. The issue of armor piercing effectiveness between the .50 and the 20mm has been the topic de jure on this forum before. Bottom line - A 20mm AP projectile has superior armor piercing ability vis a vis the .50 AP at ALL angles. In fact, as the angle increases, the margin of superiority over the .50 increases.

I suggest you read through the "Armor Penetration - 20mm vs. .50 cal." thread where "Lunatic" discusses the absolute armor piercing superiority of the 20mm over the .50 cal. at all ranges and at all angles. I suspect that given your predilection for favoring "Lunatic" as a definitive source, you will be swayed by his reasoning.

This discussion ceases to hold my interest. It is beginning to resemble the thread about the B-25 being a carrier operable aircraft. We will just have to agree to disagree.

I leave you with the following relevant British tests to mull over:

 

[delcyros]

"I leave you with the following relevant British test:"

Ahh, now You changed Your opinion and swift to the MK II with higher muzzle velocity? Ok-so far, I know the resports.
However, You cannot draw a simple conclusion from them except for that the Hispano 20mm MK II was superior at those test in comparison to other contenders under (unknown) circumstances, because:
1.) mv for ech gun is not noted (gunwear factor)
2.) true impact velocity is not noted
3.) plate penetration circumstances are unknown (the closer the impact was on the plate edges, the easier it makes to penentrate)
4.) individual plate property and condition is unknown ( with ~27 mm penetration, I suspect something like higher grade FH armour of thin thicknesses, which typically was hardened to much higher degrees than the plate hardness, I used for comparison)
5.) remaining striking velocity (post impact logic) is unknown
6.) Quality of penetration is unknown (e.g. condition of projectile behind the plate)
7.) Quantity of penetration is unknown (10 out of 12? 10 out of 10?)

M79 APCLC makes penetration more comparable because it reduces this complex matter to the physical backgrounds of the processes. Against a standart armour plate (average 1.0 Quality (220BRH), Elongation 20%), the samples are taken. Resulting impact force therefore is:
20 mm MK V (!) -true impact velocity*@ 300m: 741 m/s (2452 fps); projectile weight: 129,6 g; energy / unit hole volume: 35078 ft-lbs/in^3 (actual energy density), plate thickness: 1.56" (39,64mm), impact obliquity: 0 deg; result: penetration, ballistic limit for penetration is 2450 fps, remaining striking velocity is 89 fps for post impact logic
0.50 cal M2: -true impact velocity* @300m: 847 m/s (2803 fps),projectile weight: 48,6g; energy / unit hole volume: 41057 ft-lbs/in^3 (actual energy density), plate thickness: 1.62" (41,16mm), ballistic limit for penetration is 2803 fps, remainingstriking velocity is 35 fps for post impact logic.
And for comparison the MK II 20 mm: -true impact velocity*@300m: 784 m/s (2595 fps), projectile weight: 129,6 g.; energy / hole volume: 35997 ft-lbs/in^3 (actual energy density), plate thickness: 1.705" (43,32 mm), ballistic limit for penetration is 2595 fps, remaining striking velocity: <10fps for post impact logic.
As You see, the 20mm MK II unsurprisingly is superior to the .50 cal at even 0 deg. impact obliquity but the MK V of Your suggested layout, Jank, is inferior due to the lower initial velocity. Cut these datas in half and You get penetration figures for 400 BRH high grade FH armour, which match very well those posted in the test above.

You can do the maths on Your own, there is no cheating in them.


*) calculated with a vector speed of 400 km/h relative to target (111 m/s)

It should also be noted that M79 APCLC used the shell properties from the US M79 APC round, which is in the middle between blunt shaped and very pointed, thus slight differences may occur. However, these tend to benefit the 20mm round (at least the AP MK I), because the .50cal round has a very pointed nose, improving armour penetration at low impact obliquity ( and degrading penetration at high impact obliquities).
It should also be noted that the differences between AP MK I (20 mm) and AP MK II (27 mm) at 200m seem to reflect a completely different projectile type. I have not found anything regarding 20 mm AP MK II ammo but I suspect something like tungsten cored. Maybe Tony may help out here.

 

[Jank]

"Ahh, now You changed Your opinion and switch to the MK II with higher muzzle velocity?"

Huh? I selected the Mk. V instead of the Mk. II. To reiterate, I said,

Six Hispano Mk. V 20mm's weigh in at 554.4lbs. (I chose the Mk. V because it would have been later in the war after the P-47 assumed the air to ground role where six 20's would come in handy. If anyone prefers that I use the Mk. II 20mm instead, just add another 150lbs total weight for all six guns.)

I have no idea where you are pulling your data from but I wouldn't smell my fingers if I were you. The difference in muzzle velocity between the Mk V and Mk II is just 100fps! (830mps vs. 860mps - please see Tony's tables at WORLD WAR 2 FIGHTER GUN EFFECTIVENESS) And the projectile weight of the AP for both is the same.

In any event, Delcyros, even assuming the use of less penetrative Mk I as per the British tests, that ammunition has an advantage over the .50 at all but perfectly perpendicular strikes with AP at 200 yards. At 20 degrees, the Mk I has a 36% armor piercing advantage. At 40 degrees, the armor piercing advantage rises to 53%.

Do you want to talk about the chance that strikes will just happen to strike at perfectly perpendicular angles?

Do you want to talk about the advantages realized with the Mk II? A six gun compliment would weigh a grand total of 150lbs more! So we can go that route quite easily. That's just 75lbs more per wing over the use of Mk. V guns.

Delcyros, I just want to make sure I understand your tactical strategy in this discussion. "Lunatic" makes a statement without resort to extrinsic data for support and you draw a simple conclusion stating that, "He is reputated for knowing what he said. So believe it or not, the Skyraider suffered from recoil issues."

I offer actual data from tests carried out by the Research and Development Arm of the Ministry of Air Production and you dismiss it saying,

"However, You cannot draw a simple conclusion from them except for that the Hispano 20mm MK II was superior at those test in comparison to other contenders under (unknown) circumstances, because:
1.) mv for ech gun is not noted (gunwear factor)
2.) true impact velocity is not noted
3.) plate penetration circumstances are unknown (the closer the impact was on the plate edges, the easier it makes to penentrate)
4.) individual plate property and condition is unknown ( with ~27 mm penetration, I suspect something like higher grade FH armour of thin thicknesses, which typically was hardened to much higher degrees than the plate hardness, I used for comparison)
5.) remaining striking velocity (post impact logic) is unknown
6.) Quality of penetration is unknown (e.g. condition of projectile behind the plate)
7.) Quantity of penetration is unknown (10 out of 12? 10 out of 10?)"

In addition, I notice that while you rely in conclusory fashion on what "Lunatic" said about the Skyraider, you are apparently not prepared to rely on his opinions concerning the armor penetration superiority of the 20mm vs. the .50 cal. as set out in the other thread specifically addressing this point. Again, see the "Armor Penetration - 20mm vs. .50 cal." thread.

Also, "Lunatic" accepted the exact tests above that you are now questioning. In fact, he quoted them in his discussion on that thread. Therefore, I might add that "He is reputated for knowing what he said. So believe it or not, the 20mm is superior to the .50 in armor penetration."

You are transparent and indeed worthy of a good laugh.

 

[Tony Williams]

Armour penetration is a complex subject, and was made much more so in aircraft because a projectile would never just strike an armour plate first - it would have to work its way through the aircraft skin, structure and ancillary bits and pieces before it even reached the armour. This tended to destabilise the projectiles so that they wouldn't hit the armour point-first. The problem is that this 'destabilising effect' varied enormously from one projectile design to another. Some quotes from Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45:

Comparisons between the penetration achieved by different armour-piercing cartridges are problematic for several reasons. First, penetration figures are often quoted at different distances and striking angles (incidentally, striking angles in this work are all quoted using the convention that the most favourable angle for penetration, perpendicular (or "normal") to the armour, is measured as 90º; some other conventions have described this as 0º). Much less obviously, different qualities of armour are sometimes assumed. There have also been various national definitions of "penetration"; some (e.g. the US) specified that 50% of the shots fired have to pass through the target, others required significantly higher percentages, or do not require the shot to pass completely through the target. Finally, AP performance was usually measured on the ground. When fired from an aircraft, the actual figure would be slightly reduced in a tail chase due to increased air resistance (less of an effect at high altitude), but would be increased when firing at ground targets because the speed of the aircraft was added to the muzzle velocity, and considerably increased in a head-on attack.
In addition to these problems, the actual performance of AP projectiles in battle can vary considerably from that achieved on test. In particular, passing through the thin aluminium aircraft skin can induce yaw, i.e. disturb the straight flight of the projectile away from travelling point-first, so that it fails to hit the armour head-on, thereby significantly reducing its penetrative abilities. In fact the degree of obstruction caused by aircraft structures is such that armour much thinner than theory indicated would be necessary was often found to give satisfactory results.
The most exhaustive tests during the Second World War seem to have been carried out in Germany. Official penetration curves for 7.92 mm AP rounds tested in 1942 are worth examining in detail because they reveal the variations which can occur. The SmK-v (Spitzgeschoss mit Stahlkern verbessert; improved pointed bullet with steel core) achieved the following:
at 100 m: 12 mm / 90º, 8 mm / 60º, 3.5 mm / 30º
at 300 m: 9 mm / 90º, 6 mm / 60º, 3 mm / 30º
at 600 m: 6 mm / 90º, 3.5 mm / 60º, 1.5 mm / 30º
However, if the bullet first penetrated a 3 mm dural (light alloy) aircraft skin angled at 70º, armour penetration at 100 m dropped to 4 mm / 90º, 3 mm / 60º and 2.5 mm / 30º.
A different 7.92 mm AP loading, the Pz-v (Panzerdurchschlagsleistung verbessert; improved armour penetration) penetrated less than 11 mm / 100 m / 90º but was much more tolerant of unfavourable circumstances, achieving 9 mm / 60º and 4.5 mm / 30º. Even more significantly, after penetrating the same angled dural skin the Pz-v could still penetrate 8.5 mm / 90º, 7 mm / 60º and 3 mm / 30º. Clearly, bullet design made a huge difference.
These results are supported by tests carried out by the British in January 1941 to compare British and German rifle-calibre steel-cored AP ammunition. The performance of the .303" (11.28 g at 735 m/s) and the 7.92 mm (of unspecified type, but measured at 11.53 g at 788 m/s) was first tested against "homogeneous hard armour". The thickness necessary to achieve immunity from this ammunition at 183 m was 12.0 mm for the .303", 12.5 mm for the 7.92 mm, when striking "at normal" to the armour (i.e. at 90º). The British ammunition was significantly worse when the striking angle changed to 70º; only 6.6 mm was needed for immunity in comparison with 8.9 mm to protect against the German round.
The test then changed to shooting at the rear of the long-suffering Bristol Blenheim at the same distance, involving penetrating the rear fuselage before reaching the 4 mm armour plate protecting the rear gunner, which was angled at 60º to the line of fire. The results in this case were reversed; 33% of the .303" rounds reached the armour and 6% penetrated it. In contrast, only 23% of the 7.92 mm bullets reached the armour, and just 1% penetrated. The British speculated that the degree of stability of the bullets (determined by the bullet design and the gun's rifling) might have accounted for these differences.

and:

Continuing with the German ammunition (for which more detailed records seem to survive than any other nation's during this period) it is instructive to compare some of the standard AP types in different calibres. The little 13x64B Pzgr for the MG 131 was capable of penetrating 17 mm / 100 m / 90º but this dropped to just 10 mm / 60º and 4 mm / 30º (equivalent figures at 300 m being 11 mm / 90º and 7 mm / 60º). On the other hand, the consequences of hitting the dural plate first were negligible, only reducing penetration by 1 or 2 mm. The 15 mm Pzgr for the MG 151 at 300 m penetrated 25 mm / 90º, 19 mm / 60º and 12 mm / 30º, but the effects of the dural plate were catastrophic, with figures dropping to 11.5 mm / 300 m / 90º. Clearly, this projectile was not very stable and yawed heavily on passing through the light alloy. This may have been because it was significantly heavier than the usual HEI; the barrel's rifling twist needs to be steeper for a heavy projectile, so it could be that the designers opted for an intermediate twist which only just stabilised the AP. The tungsten-cored 15 mm Hartkernmunition achieved an impressive 42 mm / 300 m / 90º, dropping sharply to 20 mm / 300 m / 60º and 10 mm / 300 m / 30º (a characteristic of the German Hartkern designs). Figures are not given for the effect of dural, presumably because this special ammunition was supposed to be reserved for attacking armoured vehicles only.

Before turning to the 20 mm cannon it is worth mentioning the performance of the .50" Browning. The official requirement for the M2 AP was to penetrate 22 mm steel at 183 m (the M8 API was expected to match this figure at 92 m). The striking angle is not specified but is assumed to be 90º. Official US tables for the M2 show penetration at 300 m as follows: 21 mm / 90º, 13 mm / 60º and 5 mm / 30º. These measurements were to the USN criterion which called for 50% of shots to penetrate. British tests at 183 m determined that the M2 would penetrate 21 mm at 0º angle of yaw (i.e. the bullet was flying perfectly straight) but this dropped to 15 mm with only 10º of yaw (such as might be caused by passing through an aircraft's skin before hitting the armour). Further tests at 90 m firing through a heavy bomber fuselage demonstrated an ability to penetrate between 14 and 20 mm when firing at angles of between 57 and 77º. German tests credited the M2 with a penetration of 25 mm /100 m / 90º, falling to only 10 mm after passing through the dural skin (the 12.7 mm Berezin API was even worse affected, falling from 25 mm to 8.5 mm). Taking the effects of typical ranges, striking angles and fuselage structures into account, it seems likely that the practical penetration of either the M2 or M8 was in the region of 10-15 mm in normal circumstances. This was generally adequate for dealing with aircraft armour although it should be noted that Finnish pilots using Brewster Buffalos armed with .50" Brownings reported considerable difficulty in shooting down Soviet fighters from directly behind and recommended attacking from a slight angle in order to fire past the rear armour plate.

and:

The Allied 20 mm Hispano was significantly more powerful than the MG 151/20, but little attempt seems to have been made to exploit this in terms of armour penetration. As already described, the RAF settled on a mixture of HEI and a SAPI projectile which was specified to penetrate 20 mm / 200 m / 90º and ignite petrol in cans behind the plate. This was regarded, with some justification, as entirely adequate to deal with enemy aircraft. The US M75 AP shot for this cartridge was claimed to penetrate 18 mm homogenous plate or 16.5 mm face-hardened armour at 457 m / 70º. Another US official source gives penetration at 300 m as follows: 31-39 mm / 90º (depending on the armour hardness), 19 mm / 60º and 10 mm / 30º (presumably for average armour hardness). The Germans surprisingly credited this loading with only 25 mm / 100 m / 90º, although noted that passing through the usual dural skin only reduced this to 21 mm. The British did experiment with a tungsten cored type, similar to Hartkernmunition, (the AP Mark III.z) designed by Janecek of "squeezebore" gun fame, which was intended to penetrate up to 60 mm / 200 m / 70º, but it seems that it was ballistically unsatisfactory and was not adopted.

Tony Williams: Military gun and ammunition website and discussion forum

 

[delcyros]

I have read that it was a simple task to reduce a guns ROF which increased relaibility and reduced wear on the barrel. If the ROF was reduced to 600 rpm for the twenty milimeter, then the firing time would be increased from 14 seconds to 17.5 seconds which is very respectible. Less jams too. The total output would still be 3,600 ROF per minute which would be more than the 3,000 ROF per minute for a four gun armed 20 milimeter set up.

True. The volume of fire per sec. would have been drastically reduced in this layout to 60 rps in comparison to 100 rps with eight 0.50cal. Still, 60 rps is impressive for cannon fire.

 

[delcyros]

Huh? I selected the Mk. V instead of the Mk. II.

I have no idea where you are pulling your data from but I wouldn't smell my fingers if I were you. The difference in muzzle velocity between the Mk V and Mk II is just 100fps! (830mps vs. 860mps - please see Tony's tables at WORLD WAR 2 FIGHTER GUN EFFECTIVENESS) And the projectile weight of the AP for both is the same.

In any event, Delcyros, even assuming the use of less penetrative Mk I as per the British tests, that ammunition has an advantage over the .50 at all but perfectly perpendicular strikes with AP at 200 yards. At 20 degrees, the Mk I has a 36% armor piercing advantage. At 40 degrees, the armor piercing advantage rises to 53%.

You are transparent and indeed worthy of a good laugh.

To this point I enjoied the discussion but now I see no justification to continue (with the attitude You have) so this is my last post in this thread.
The difference between MK-II and MK V is due to true impact velocity in the comparison I made. I actually calculated with a 40 m/s difference according to:
The WWII Fighter Gun Debate: Gun Tables
And if You check this with the true impact velocities for MK V and MK II, the difference at 300 m is only 43 m/s according to Balltraj. The weight of both AP-rounds is the same, You don´t need to worry about this.
From Tony´s 20mm datas:"Another US official source gives penetration at 300 m as follows: 31-39 mm / 90º (depending on the armour hardness)".
This almost exactly match my calculations, altough now I am confident that 220BRH hardness for such thin plates is highly improbable. Thin plates can be treated better than thick ones and usually are hardened to high degrees:
typical BRH values for (homogenious) target plate thicknesses (in mm):
5 to 15------435 to 465
16 to 30-----338 to 382
31 to 50-----323 to 368
51 to 80-----309 to 338
81 to 120----279 to 309
121 to 150---235 to 265
151 to 275---206 to 235
(all from german sources)
typical BRH values for homogenious target plates (in mm):
6 to 13------330 to 370
25-----------240 to 350
38-----------240
51-63--------240
76 to 127----220 to 240
>127---------220
(all from US sources)
typical BRH values for homogenious target plates (in mm)
3 to 30------440 to 475
15 or more---300 to 331
3 to 14mm---340-388
15 to 30-----262-321
31 to 80-----262-321
85 to 120----255-302
125 to 160---241-285
>160--------?
(all from british sources)
The underlined figures are those relevant to the tests.
This may explain the significant differences between the tests to some degree. Esspecially for british tests it is impossible to draw conclusions if those plate properties are not given because several, different specifications (four!) were used contemporary for target plates.
Now if You factor the different 20mm AP results from Tony:
from german test: 25 mm / 100 m / 90º (impact obliquity= 0 deg) [BRH= 338-382]
from british test: 27 mm /200 m / 0 deg [BRH= [U]262-475[/U]]
from US tests: 31-39 mm/ ? / 90º [BRH = 240-350]
None of the tests is comparable to each other since the plate hardeness remains unknown. The german target plates differ less in hardness for this thickness than do US plates and particularly british plates (due to different specifications).
The M79 APCLC results from me give results for the same plate hardeness, so a comparison between the guns is possible (all using AP-rounds):
20 mm MK II: 43,3 mm / 300 m / 0 deg [BRH= 220]
20 mm MK V: 39,6 mm / 300 m / 0 deg [BRH = 220]
.50 cal M2: 41,1 mm / 300 m / 0 deg [BRH = 220]

There is nothing wrong with my datas, except for the plate hardness, which seems to be too soft, hence the deifferences to the recorded datas. I also suspect the germans used something more around BRH 380 for their tests, while the british actually may used something in the region between 300 and 350 BRH. The US plates, which match my calculations most are also made from comparably soft material (BRH 240-350, depending on hardness, as the quote imply). I cannot be sure here as long as the plate properties are not given, but the calculations give an idea what happens. Better armour penetration tests do not give the plate thickness, which is defeatable (this is dependent on more factors) but the minimum necesaary striking velocity for a specific projectile at a certain impact obliquity to defeat the plate, which also always is charackterized regarding ductility and hardeness (both is missing in these records).
There is no doubt that the MK V (unlike the MK II) has no advantage over the 0.50 cal in penetration abilities at 300 m and 0 deg impact obliquity. That´s what I said. I do also confirm that the superiority of the 20mm round is significant as the impact obliquity rises. The tungsten cored and tungsten tipped AP rounds for 20mm and .50cal. respectively, do have different plate penetration curves.
By the way, thanks for the datas, Tony, esspecially the yaw inducing datas are interesting.

best regards,

 

[Tony Williams]

The M79 APCLC results from me give results for the same plate hardeness, so a comparison between the guns is possible (all using AP-rounds):
20 mm MK II: 43,3 mm / 300 m / 0 deg [BRH= 220]
20 mm MK V: 39,6 mm / 300 m / 0 deg [BRH = 220]
.50 cal M2: 41,1 mm / 300 m / 0 deg [BRH = 220]

I have penetration curves for the .50 Cal AP M2 and the 20mm AP M75, both from the Ballistic Section, Technical Division, USN, and both dated December 1943. We may therefore assume that the data is comparable.

The .50 Cal was measured against homogenous plate of 350 BHN. At 300m this gave the following figures:
Striking velocity (from aircraft gun: 36" barrel) = 740 m/s.
Penetration at 0 degrees = 22 mm

The 20mm was fired against plates of various hardnesses. Against 350 BHN plate, the following figures are given:
Striking velocity at 300m = 655 m/s
Penetration at 0 degrees = 39 mm

Tony Williams: Military gun and ammunition website and discussion forum

 

[Squeelig]

Wouldn't the effect of yawing have a greater effect on the fifty's ability to penetrate than the twenty milimeter?

I still don't see how any of this is relevant guys. You still have a situation where each round of twenty milimeter is at least 2.5 times as effective as each one from a fifty.

 

[Jank]

I think the loss of velocity that Delcyros listed at 300 meters is frankly too little. A start of 880ms and still travelling at at 847ms at 300 meters? The .50had a great BC but there would be more velocity loss. That much I know.

The differences in striking velocity at 300 meters between the data that Delcyros used and Tony used is as follows:

For the .50, Delcyros used 847ms and Tony used 740ms

For the 20mm, Delcyros used 741ms and Tony used 655ms.

Tony said,"The .50 Cal was measured against homogenous plate of 350 BHN. At 300m this gave the following figures:
Striking velocity (from aircraft gun: 36" barrel) = 740 m/s.
Penetration at 0 degrees = 22 mm

The 20mm was fired against plates of various hardnesses. Against 350 BHN plate, the following figures are given:
Striking velocity at 300m = 655 m/s
Penetration at 0 degrees = 39 mm"

I take it that we have closure now. The 20mm has a penetration superiority on the order of 77%. At angles of deflection, that superiority would increase.

It is simply not possible that, pursuant to Delcyros' assertion, increasing the velocity by 107ms for the .50 and 86ms for the 20mm (a 20ms difference in increase between the two) would then result in the .50 now outpenetrating the 20mm.

Besides, "Lunatic" would disagree with you. That settles it.

 

[Jank]

Squeelig said, "Wouldn't the effect of yawing have a greater effect on the fifty's ability to penetrate than the twenty milimeter?"

Good point. Yes. The .50 has greater sectional density than the 20mm and given its comparatively low weight, relies on that for penetration so yawing would indeed be more detrimental to the penetration of the .50 than the 20mm.

You will also notice that the .50 is affected by angles of deflection much more than the 20mm.

I think that ideally, comparing perpendicular hits (90 degrees or 0 degrees depending on how you figure it) is unrealistic. All comparisons should be at 45 degrees. Comparing penetration at 90/0degrees gives a false boost to the .50 cal's armor penetration abilities under circumstances that would be a rarity in the real world of combat. Even Delcyros' data wouldn't support an outcome where the .50 is superior to the 20mm at 45 degrees.

For an illustration of the effect on penetration due to angle of deflection between the .50 and 20mm, look below. Looking at the Mk I vs. .50 tests, we see 100% parity at 90/0 degrees, then at 20 degrees, the .50 is down to 70% and the 20mm is down to 95%, then at 40 degrees, the .50 is down to 40% and the 20mm is down to 75% of it's ability to penetrate. The 20mm retains penetrative ability at angled strikes far better than the .50 does. (This is Mk I and not the Mk II)

And why just examine armor penetration? Look at the penetration through sand. The .50 at 200 yards penetrates 15" while the 20mm Mk I penetrates 24" (a 60% increase over the 50's ability to penetrate). (This is Mk I and not the Mk II)

 

[Jank]

Delcyros,

As I said above, I think the loss of velocity that Delcyros listed at 300 meters is frankly too little. A start of 880ms and still travelling at at 847ms at 300 meters? The .50had a great BC but there would be more velocity loss. That much I know.

From "Lunatic's" website at Lunatic's WWII Aircraft Gun Ballistics Page

Note that the .50 BMG is listed as having a velocity of 736ms at 300 meters and not the 847ms figure you are using.

As you well know, Lunatic knows what he's talking about which, of course, necessarily means that you don't know what you're talking about.

Lastly, it just so happens that Tony's data matches Lunatic's. This makes perfect sense because Lunatic bases his information on Tony's data. Tony lists 740ms at 300 meters.

Again, Tony's data states:

The .50 Cal was measured against homogenous plate of 350 BHN. At 300m this gave the following figures:
Striking velocity (from aircraft gun: 36" barrel) = 740 m/s.
Penetration at 0 degrees = 22 mm

The 20mm was fired against plates of various hardnesses. Against 350 BHN plate, the following figures are given:
Striking velocity at 300m = 655 m/s
Penetration at 0 degrees = 39 mm

Even if the data above was realized with Mk. II as opposed to Mk. V, (difference at the muzzle of less than 35ms) that wouldn't diminish the penetration at 300m by more than 17 mm (a reduction of 17mm is 2/3 of an inch and would represent a 43% decrease) which would be required in order to put it at below the .50's ability to penetrate 22mm.

 

[delcyros]

Delcyros,

As I said above, I think the loss of velocity that Delcyros listed at 300 meters is frankly too little. A start of 880ms and still travelling at at 847ms at 300 meters? The .50had a great BC but there would be more velocity loss. That much I know.
Note that the .50 BMG is listed as having a velocity of 736ms at 300 meters and not the 847ms figure you are using.

The 20mm was fired against plates of various hardnesses. Against 350 BHN plate, the following figures are given:
Striking velocity at 300m = 655 m/s
Penetration at 0 degrees = 39 mm[/b]

Even if the data above was realized with Mk. II as opposed to Mk. V, (difference at the muzzle of less than 35ms) that wouldn't diminish the penetration at 300m by more than 17 mm (a reduction of 17mm is 2/3 of an inch and would represent a 43% decrease) which would be required in order to be below the .50's ability to penetrate 22mm.

Dear Jank, dear Tony,

The impact velocities are correct, as I stated above:
"*) calculated with a vector speed of 400 km/h relative to target (111 m/s)"
so I used Lunatics values for 300 m (for example: 20mm MK II: 673 + 111 = 784 m/s; .50cal: 736 + 111 = 847 m/s) to get a true impact velocity (including plane vectoring speed, the gun is not fired with speed zero). Altough all will be a bit lower due to the higher initial rate of deceleration. Note that the 20mm round even benefits from this (!) and puts the .50 cal to a disadvantage, factoring the higher BC (= less deceleration) of the 0.50 cal rounds. I would be glad to have a computing system for deceleration rates of small calibre rounds, but sadly I do only have them for medium to large calibre rounds (50 mm and up), so these are approximations but very close ones, relied on Lunatics page (whom´s calculation I trust unless You present better results).

Tony, can You give me more datas or a copy about this specific test. All I have studied so far implies that 39 mm @ 300 m / 0 deg with a plate hardeness of 350 BRH is totally contradicting even british datas and sources (not to speak of other US and german tests). A testplate of 39 mm is quite difficult to process with a specification calling for 350 BRH, hence specifications do call for 240 BRH for these thickness and US tests. I know there are plates of this hardeness used by germans but for the US I know only two experimental plates, which match those requirements, and none of them actually was tested against 20mm rounds. I am not aware of other special plates used for .50 cal tests on prooving grounds (it may have happened), so this source would interest me in particular. Indeed such a figure would surprise not only me but also friend and expert Dr. Wegener. In case this plate indeed has 39 mm and 350 BRH and has no major internal flaws (bubbles, pipings (long tube-like bubbles), cracks, laminations, and any other irregularities, this would exceed even the special treated plates with about 300 Brinell (32 Rockwell "C") hardness for plates under 51mm thickness!
This penetration value is also totally off-line if I use the official Ministery of Defense british penetration formula for homogenious armour, announced 1942:
B0 = 500 - (160)(0.434294482)[LOGe(D / 1.5648 )]

B1 = 54000 / (B0 - BHN)

FOB1 = 11800 / (65o - OB)

FOB2 = (43.4)(PQF)(T/D)[SQRT(BHN)]/COS[(1.5)(OB)]

FOB3 = 929 - B1 - FOB1 + FOB2

BRITNBL = FOB3 / SQRT[W/D3]

But it fits very well with a plate hardeness of 250 BRH (.8% difference).
This would indeed imply an actual energy density above 65000 ft-lbs/in^3, which is hard to believe (impact force necessary to cause such energy densities would even be higher than a 37 mm NS37 impact from this distance!).

Thanks in advance,

 

[Jank]

"The impact velocities are correct, as I stated above:
"*) calculated with a vector speed of 400 km/h relative to target (111 m/s)"
so I used Lunatics values for 300 m (for example: 20mm MK II: 673 + 111 = 784 m/s; .50cal: 736 + 111 = 847 m/s) to get a true impact velocity (including plane vectoring speed, the gun is not fired with speed zero)."

You are correct and I am an ***.

I am still not convinced that the .50 cal would out penetrate a Mk V 20mm at 300 meters though. Perhaps Tony can give his thoughts.

And what about at a 20 degree angle? What about at a 40 degree angle? As I indicated above, pependicular strikes would be rare. Tony indicated that after striking the aluminum skin, the bullet yaws thereby not striking with the maximum ability to bring its sectional density to bear. And lastly, the penetrative ability of the .50 would be affected more by both yaw and angled strikes than the 20mm.

Additionally, when the USN determined that ther 20mm was 2.5 times as destructive at longer ranges, how exactly did they define destructive? Even the Mk II doesn't have 2.5 times the armor penetration ability so I believe that were referring to effects beyond mere armor piercing ability. Whatever effects they include, they would favor the 20mm over the .50.

And finally, the 6 x Mk V setup can still be swapped out for a 6 x Mk II setup at a total increase in weight of 150lbs for all six guns. As you may recall, I originally advanced the idea of using Mk V instead of Mk II because you were concerned with weight degrading the flight performance of the Thunderbolt. I always maintained that it wouldn't make a hill of beans difference, especially in the ground attack role.