20/20 Hindsight - different armament?

I actually liked the idea with the MK V due to their lower recoil , weight higher rof. The long barreled MK II with recoil-damping gunmount might offset the larger brutto recoil as I learned from Jank, but it still is a very bulky weapon.

Regarding higher impact obliquities, the 0.50 penetration curves drops with a steady rate, much steeper than the 20 mm. This may be partly caused by the thickness / diameter relation and mostly because of the blunter nose of 20 mm projectiles. I can perform calculations, if You desire (all factors beeing the same as above=300m, 220 BRH):
20mm MK II: 41,6mm(20deg), 37,5mm(40deg), 33,4 mm (45 deg)
Note that it takes only a loss of 4 mm between 20 and 40 deg and further 4mm between 40 and 45 deg. impact obliquity for the 20mm AP MK I fired from 20mm HS MK II.
0.50 cal AP: 39,1mm(20deg), 33,0mm(40deg), 28,9mm (45deg)
Note that the .50 cal projectile loses 6 mm penetration between 20 and 40 deg. impact obliquity and further 6 mm for 40-45 deg. impact obliquity.
Loss is relative to armour quality. The 220 BRH are too low, as we recognized, so I estimate that the .50 cal round would have more detremental losses with higher grade armour material than this softer "comparison steel". Also, the less pointed nose of the 20mm round will perform better at high impact obliquities (program credits only for a standart middle shaped nose), which is not reflected by the calculation above (the significance rises with impact obliquity).
Yawing effects are not that important for ground attacks, since few vehicles (unlike planes!) had layered armour layout (or inner mounted plates). They are not quantifyable according to recent knowledge. They do happen and this is expressed with an "estimated percentage of Yaw" rather than with "a loss of penetration" -figure, altough Tonys infos covering this aspect are highly interesting. Too much factors play a role: impact velocity, plate 1 thickness, plate 2 (..3...4) thickness, angles, spacial distribution (the larger the distance beween the more probable are yawing effects), spin of projectile and weight (=stability border to induce yaw) and much others. Yawing effects can be considerable but more "stable" projectiles (not necessarely heavier ones) need more inducing force for them.
There is no doubt that the 20mm round carries more HE charge, is heavier, has more source for fragmentation and therefore will cause more damage against air targets and "soft" ground targets.

What would the armor penetration be for a .50 vs a 20mm fired from a Mk V at 300 meters at 20 degrees deflection? At 40 degrees deflection?

...38,0mm at 20 deg. with a similar 4mm dropping rate to the MKII with 33,9mm at 40 deg and 29,8mm at 45 deg. (true values should be slightly better due to less pointed nose with higher impact obliquity, note). 0.50cal as above, so there is parity in penetration at around 30 deg (probably more close to 25 deg.) and beyond this the 20mm MK V, despite the lower muzzle velocity, has a better penetration according to M79 APCLC.

A few comments:

The USN penetration graphs to which I referred I have only in the form of poor-quality photocopies. I do not know how the results were obtained, but I can hardly imagine that they had a huge variety of plates of different thicknesses and fired at each one at every range and striking angle - I expect that they carried out a few actual tests at critical points and interpolated the rest (that would be the only practical way to do it IMO).

I should emphasis that the 20mm AP tested was the US M75, which was different from any British rounds. It was solid steel, weighed 165 g and was fired at 775 m/s. The British developed several Marks of 20mm AP, but the RAF appeared to make relatively little use of them; the most common seems to have been the Mk II, which had a hollow cavity and weighed 141 g (I don't have a muzzle velocity figure, but it was presumably around 820-850 m/s from the Mk II).

The penetration figures most often quoted for the British 20mm refer to the SAPI, which was the hollow HE shell filled with incendiary material and fitted with a hardened steel nose-cap instead of a fuze. This had the advantage of matching the trajectory of the HEI, and the two types of round were usually mixed in equal numbers in the ammo belt. Obviously, the penetration of the SAPI would not have been as good as a solid AP. I don't have directly comparable figures, but the penetration of the SAPI at around 20-25mm was similar to that of the .50 AP - i.e., good enough to deal with aircraft armour. Incidentally, I have never seen any penetration claims for the .50 AP which exceed 25mm armour plate, at any distance or striking angle.

I also have penetration graphs for the German 20mm AP (which also had small cavities, often filled with incendiary material), these show that the low-velocity MG-FF (117 g at 585 m/s) would penetrate 18mm at 300m (against 150 kg/mm2 armour), reducing to 11mm after penetrating a 3mm dural skin at 70 degrees. The MG 151/20, which fired the same proj at just over 700 m/s (v around 530 m/s at 300m), penetrated 23mm (or 12mm after passing through the dural). Obviously, with similar projectiles the Hispano would penetrate significantly better than this, as the cartridge was far more powerful.

The USN charts for the fall-off with striking angle show that the .50 AP (which gives 22mm/300m/0 degrees) sees a fall-off to 17mm at 20º, 13mm at 30º, 11mm at 40º, 9mm at 45º, 7mm at 50º and 4.5mm at 60º.

The USN chart for the 20mm AP is less helpful, because although it shows penetration at 0º for 300, 350 and 400 BHN plate (46mm, 39mm and 32mm respectively), it only gives one figure for other angles. However, it is reasonable to assume that these figures probably relate to 350 BHN plate as that is the middle one of the three, and the same used for the .50. Anyway, they show a sharp initial fall-off to 21mm/20º, before levelling off at 19mm/30º, 18mm/40º, 17mm/45º and 10mm/60º.

Tony Williams: Military gun and ammunition website and discussion forum

Tony said, "I should emphasis that the 20mm AP tested was the US M75, which was different from any British rounds. It was solid steel, weighed 165 g and was fired at 775 m/s."

Geez, that's like 6 ounces!

Tony Williams said:

I expect that they carried out a few actual tests at critical points and interpolated the rest (that would be the only practical way to do it IMO).
(...)I have never seen any penetration claims for the .50 AP which exceed 25mm armour plate, at any distance or striking angle.

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It´s not surprisive. With a plate QF:1.0 and BRH 220 there will be no target plates of such thin thicknesses to test vs. .50cal impacts. A BRH of 300-400 is more plausible, ergo a plate QF of between 1.2 an 1.4. For such properties and under assumption that the gun does not move (unlike an airplane), I get fairly comparable results for the .50cal: 21,9-25,8mm at 300m/ 0 deg.
USN penetration specifications are usually very reliable sources. I understand that plates have been penetrated under controlled circumstances and most records do also give a remaining striking vel. for the projectile (I suspect but am not sure, that this was recalculated via distance travelled behind the plate either in air or through sand). So actually there was no 39 mm plate with 350 BRH but (probably) a usual 1" plate of 350 BRH. The penetration at 300m distance was recorded and the remaining striking velocity was calculated back to reach 39mm with the M75 20mm AP for Navy ballistic limit of full penetration. This was common practice also for larger projectiles, which are more familar to me.
With the update in the datas (heavier projectile, striking vel), I still have problems to recall this result via M79 APCLC:
M75 20mm AP(165 g plain steel), impact vel.: 655m/s, plate QF: 1.1 (equals 280 BRH), obliquity: 0 deg; max. penetration: 1.41" (35,8mm)
plate QF: 1.3 (equals 350 BRH): max penetration: 1.19" (30,2 mm), so either the projectile performs much better than estimated or the program has severe failures or the plate does not fit to the specification calling for 350 BRH. There is still a significant gap in energy density, something to keep in mind.

I think that for a fighterbomber or heavy fighter, HE and AP performance is vital.

I have often thought that the P47 should have had 4 Hispano 20mm's instead of 8 .50's. Instead of another pair of 20mm's, I'd have extra ammo or fuel.

Would it be possible to have a 40mm, or even 57mm, under each wing of a P47?

I know America couldn't make the Hispano work though, so it's pie in the sky.

...to the debate on gun recoil any more, but I came across some new and relevant information while looking for something else (which is usually the way )

Tim Mason's book "The Secret Years - Flight Testing at Boscome Down 1939-45" is concerned with the official tests of new or modified aircraft to ensure that they were ready for service. Boscome Down was the home of the "Aeroplane and Armament Experimental Establishment" (or A&AEE), which tested every aspect of the planes' performance, handling, and armament. The book summarises the results of the tests, picking out points of interest.

One of the planes tested was Hurricane Mk 1 L1750, which in late 1939 was fitted with two Hispano cannon in underwing gun pods (obviously a less aerodynamic solution than the later in-wing installation). The book contains this comment: "Aircraft handling remained unchanged, although firing one gun induced slight yaw; performance suffered only marginally."

Several Hurricane Mk IIC were tested, as they tried to 'debug' the cannon installation. There were only two complaints concerning the cannon: that the ammo capacity of the original drum-fed version was inadequate, and that the guns froze up at altitude - which proved difficult to solve as they couldn't get enough hot air to them.

Some Spitfires with four 20mm cannon were tested. The installation was regarded as satisfactory, the main comment being that the installation added 200 lb (91 kg) and shifted the CG rearwards, required a 7.5 lb inertia weight. Four cannon also proved more difficult to keep warm than two plus MGs (I have read before that this was a key reason for not fitting Spitfires with four cannon - the guns froze at altitude). This problem took until mid-1944 to solve - although it then returned with the Griffon-engined versions as the engine ran much cooler than the Merlin so didn't provide as much gun heating. It is stated that the Hispano+Browning combo was preferred because the Browning was much less prone to freezing up.

The Spitfire XXI seems to have resolved these problems, the only comment about the four 20mm cannon being that they gave "accurate shooting from the steady platform".

To sum up, there is not a single reference to the recoil of the cannon causing any issues with either the Hurricane or the Spitfire, except for the comment that firing just one gun in the Hurricane caused "slight yaw". The main reason for the RAF avoiding the four-cannon layout was the gun freezing problem at altitude, which was tolerable in the Hurricane as it soon became used for ground attack anyway. It is probably significant that the only reference I can recall to the use of four-cannon Spitfires was in the Mediterranean theatre, where ambient temperatures were higher.

Tony Williams: Military gun and ammunition website and discussion forum

Thanks Tony. These reports on four 20mm installations in the Hurricane and Spitfire indicate that a six 20mm installation in a P-47 would not have presented recoil related issues of any significance.

Thanls for the follow up.

To my mind the question is round the wrong way. The US managed perfectly well with the 0.5" machine gun till the korean war. Perhaps we should be wondering if the RAF got it right with opting for eight 0.303"? Wouldn't four or six 0.5" have been better? Come the Battle of Britain, such puny rifle calibre guns were only saved by the Dixon (usually mistakenly referred to as De Wilde) ammunition.

"Perhaps we should be wondering if the RAF got it right with opting for eight 0.303"

That was funny.

merlin said:

Perhaps we should be wondering if the RAF got it right with opting for eight 0.303"? Wouldn't four or six 0.5" have been better? Come the Battle of Britain, such puny rifle calibre guns were only saved by the Dixon (usually mistakenly referred to as De Wilde) ammunition.

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Aircraft went through a process of up-armouring as a result of battle experience during 1940. At the start of the fighting, eight .303s were probably the best armament of any fighter in service. As aircraft toughened up, so .303s became less effective. By the end of the BoB, a quartet of .50s would probably have been better (although heavier). Even so, the RAF continued fitting .303s to fighters for several years, so they can't have regarded them as useless.

The B Mk VI ("De Wilde") incendiaries were a definite improvement on any other type at the time, but they weren't that common initially. See my analysis of BoB armament HERE

The RAF had realised that the .303 would no longer be adequate as soon as planes started to adopt armour plate, and were planning as early as 1935 for fighters armed with 20mm cannon - but the Hispano took longer than expected to de-bug, so it just missed contributing to the BoB.

Tony Williams: Military gun and ammunition website and discussion forum

It wasn't just the advent of armor plate that rendered the .303 less than adequate but the structural and other "hard" components of the aircraft itself.

A .303 with Mk. I AP would penetrate just 5mm of armor plate at 200 yards at 20 degree deflection. A .50 AP round under the same circumstances would penetrate 14mm. Even at 600 yards, the .50 under the same circumstances could still penetrate 9mm of armor plate. Obviously, it was far easier to critically damage your opponents engine with a few .50's than a dozen or more .303's.

Yes, obviously the Brits didn't regard it as useless but their ongoing use of it does not mean it was effective relative to other available armaments like the .50 either. Case in point. The USAAF persisted in using the .50 on its F-86 when it would have been far more effective with even three 20mm's. Many, many Migs made it back to base peppered with .50 rounds.

Jank said:

It wasn't just the advent of armor plate that rendered the .303 less than adequate but the structural and other "hard" components of the aircraft itself.

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I agree. The RAF were also aware of this, which is why they decided in the mid-1930s that they needed to fit their fighters with eight of the guns, at a time when most fighters in service only had two. They were working on the calculation that a high volume of hits would compensate for the low effectiveness of each one, but this was not entirely successful.

The Luftwaffe also perservered with rifle-calibre guns, of course, with the 109 and 190 carrying 7.9mm cowling guns until about 1943 IIRC.

Tony Williams: Military gun and ammunition website and discussion forum

Interesting discussion guys.

As to armor penetration and the effect the size of the incoming projectile has is here explained by Robert D. Livingstone, Co. author of "World War II Ballistics: Armor and Gunnery".

"Armor obliquity effects decrease as the shot diameter overmatches plate thickness in part because there is a smaller cylindrical surface area of the displaced slug of armor which can cling to the surrounding plate. If the volume which the shot displaces has lots of area to cling to the parent plate, it resists penetration better than if that same volume is spread out into a disc with relatively small area where it joins the undisturbed armor. Plate greatly overmatching shot involves the projectile digging its own tunnel, as it were, through the thick interior of the plate. It was found experimentally that the regions in the center of the plate produced the bulk of the resistance to penetration, while the outer regions, near front and rear surfaces, presented minimal resistance because they are unsupported. Thus, an overmatched plate will be forced to rely on tensile stresses within the displaced disc, and will tend to break out in front of the attacking projectile, regardless of whether the edges cling to the parent material or not. Plate obliquity works in defeating projectiles partly because it turns and deflects the projectile before it begins digging in. If there is insufficient material where the side of the nose contacts the plate, stresses will travel all the way through the plate and break out the unsupported back surface. The plate will fail instantaneously rather than gradually".

"You can angle the armor any way you want, and beyond a certain point of shot overmatching plate, the obliquity will cease to be relevant. In fact, at certain conditions of shot overmatching plate, the cosine rule is broken and the plate resists less well than the simple cosine relationship would predict (LOS thickness is greater than effective thickness). The above only applies to WWII era AP and APC/APCBC, and WWII sub caliber ammunition. The long rod penetrators of today are greatly overmatched but they bring so much energy to the plate that they penetrate by "ablation" in which both projectile and armor behave like fluids. Hollow charge also enters the field of fluid dynamics, with a very thin jet penetrating overmatching armor with ease, regardless of obliquity"

Was he's talking about here is the effect of the T/D coefficient between the incoming projectile and the armour it opposes.

I note in an earlier reply Tony referred to the "De Wilde" incendary shells for the 0.303" machine guns. Isn't it about time the De Wilde reference was dropped - it was only retained to fool the Germans! The real hero was Captain Dixon, who realised that the De Wilde shell being hand made without any 'measured' amount ingredients, couldn't be mass-produced. It was fortunate that he unofficially conducted tests until he came up with the solution - which was passed on the the Americans!
Hence he deserves the credit for his achievement.

Sorry for being slightly off-topic!!

That is quite true, but they are still popularly known as "De Wilde" bullets. if I want to be technically precise, I refer to them as the B Mk VI (superseded by the B Mk VII), which is their official designation.

Earlier in this thread someone mentioned an English ace who always checked to see if he was in a turn before firing his guns. I believe that was Stanford Tuck and he was not checking whether in a turn but checking whether he was in a skid or uncoordinated flight. I also believe that the eight gun armament of the British fighters was partly because of the Tactics they were using and because of the poor marksmanship of the pilots. They theorised that they would open fire at long ranges with a lot of guns and get hits because of the many bullets in the air. Sort of a shotgun approach. Also I don't know if this has been mentioned but a significant number of Corsairs were armed with 4- 20mm cannon in WW2 and some night fighter versions of the Hellcat had 2-20mm cannon plus 4-.50 MGs.

renrich said:

I also believe that the eight gun armament of the British fighters was partly because of the Tactics they were using and because of the poor marksmanship of the pilots. They theorised that they would open fire at long ranges with a lot of guns and get hits because of the many bullets in the air. Sort of a shotgun approach.

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No - the decision to go to eight guns was taken in around 1934/5, as a result of theoretical studies which showed that the increasing speed of aeroplanes meant that a pilot may only be able to fire at a target for about two seconds (IIRC). So they did some sums to work out how much damage one machine gun could do to an aircraft in two seconds, and reckoned that they would need eight of them to do the job.

They started out with the guns harmonised for 400 yards, on the assumption that a typical attack run would start at that distance and finish off at 200 yards. They estimated that the spread of fire at short range would cover the fuselage and engines of a twin-eingined plane, so all the bullets should still be hitting. Early WW2 experience showed these figures were too optimistic, so they dropped the harmonisation range to 250 yards.

The RAF became concerned at standards of pilot accuracy in deflection shooting in the late 1930s, so started working on the gyro gunsight. That had no effect on the choice of armament though.

Thanks Tony for your explanation. I had read somewhere about the theory you discussed but was quoting from memory which was quite hazy. According to John Lundstrom in "The First Team" the European air forces did not place much reliance on deflection shooting in contrast to the US Navy perhaps partly because of visibility problems in inline engined a/c. An aside, I saw gun camera film of a German fighter firing at a P38 from the 6 o clock position about 30 degrees high. What a target with a lot of vital parts.