Armour penetration
"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 90 degrees; some other conventions have described this as 0 degrees). 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 percent of the shots fired have to pass through the target, others required signficantly higher percentages, or did 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 7.92mm AP rounds tested in 1942 are worth examining in detail because they reveal the variations wihch can occur. The SmK-v (Spitzgeschoss mit Stahlkern verbessert, improved pointed bullet with steel core) achived the following:
At 100 meters:- 12mm/90 degrees - 8mm/60 degrees - 3.5mm/30 degrees
At 300 meters:- 9mm/90 degrees - 6mm/60 degrees - 3mm/30 degrees
At 600 meters:- 6mm/90 degrees - 3.5mm/60 degrees - 1.5mm/30 degrees
However, if the bullet first penetrated at 3mm dural (light alloy) aircraft skin angled at 70 degrees, armour penetration at 100 meters dropped to 4mm/90 degrees, 3mm/60 degrees and 2.5mm/30 degrees.
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 grams at 735 meters/second) and the 7.92mm (of unspecified type, but measured at 11.53 grams at 788 meters/second) was first tested against 'homogeneous hard armour'. The thickness necessary to achieve immunity from this ammunition at 183 meters (200 yards) was 12.0 mm for the .303 in, 12.5mm for the 7.92mm, when striking 'at normal' to the armour (i.e. at 90 degrees). The British ammunition was significantly worse when the striking angle changed to 70 degrees; only 6.6mm was needed for immunity in comparion with 8.9mm 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 60 degrees to the line of fire. The results in this case were reversed: 33 percent of the .303 in rounds reached the armour and 6 percent penetrated it. In contrast, only 23 percent of the 7.92mm bullets reached the armour, and just 1 percent penetrated. The British speculated that the degree of stability of the bullets (determined by bullet design and the gun's rifling) might have accounted for these differences.
Continuing with the German ammunition (for which more detailed records seem to survive than any other nation's during the period), it is instructive to compare some of the standard AP types in different calibres. The little 13 x 64B Pzgr for the MG-131 was capable of penetrating 17mm/100 meters/90 degrees, but this dropped to just 10mm/60 degrees and 4mm/30 degrees (equivalent figures at 300 meters being 11mm/90 degrees and 7mm/60 degrees). On the other hand, the consequences of hitting the dural plate were negligible, only reducing penetration by 1 or 2 mm.
Before turning to the 20 mm cannon it is worth mentioning the performance of the .50 in Browning. The official requirement for the M2 AP way to penetrate 22mm steel at 183 meters (200 yards) (the M8 API was expected to match this figured at 92 meters). The striking angle is not specified but assumed to be 90 degrees. Official US tables for the M2 show penetration at 300 meters as follows: 21mm/90 degrees, 13mm/60 degrees, 5mm/30 degrees.
These measurements were to the USN criterion which called for 50 percent of shots to penetrate. British tests at 183m determined that the M2 would penetrate 21mm at 0 degrees angle of yaw (i.e. the bullet was flying perfectly straight), but this dropped to 15mm with only 10 degrees of yaw (such as might be caused by passing through an aircraft's skin before hitting the armour). Further tests at 90 meters firing through a heavy bomber fuselage demonstrated an ability to penetrate between 14 and 20 mm when firing at angles of between 57 and 77 degrees. German tests credited the M2 with a penetration of 25mm/100 meters/90 degrees, falling to only 10mm after passing through the dural skin (the 12.7mm Berezin API was even worse affected, falling from 25mm to 8.5mm). 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 adaquate for dealing with aircraft armour, although it should be noted that Finnish pilots using Brewster Buffalos armed with .50 in 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.
German 20mm ammunition did not have sufficient velocity to justify the use of Hartkernmunition, so the AP types were plain steel with some incendiary content, the 117 gram Pzbr containing a capsule with 3.6 grams of phopshorous. The low velocity (under 600 m/sec) MG-FFM, usually derided for its lack of penetration, was actually quite satisfactory when loaded with one of the various Pzgr projectiles which seem to have become available from 1941. Maximum penetration (100 m/90 degrees) varied between 18 and 24 mm depending on type, with the more realistic 300m/60 degree giving figures of 9-10 mm. The effect of prior penetration of the usual angled 3mm dural skin varied hugely, from about 5 percent to around 40 percent reduction, depending on the projectile type and the striking angle. As one would expect, the results for the higher velocity MG-151/20 firing the same projectiles was slightly better, at 12-13mm/300m/60 degrees. The British considered what thickness of armour was required to protect against this ammunition, and determined that for a bomber around 15-18mm would be adaquate, although for protecting the rear of the existing Spitfire and Tempest fighters the best solution within the weight limits would be two plates of 7mm and 4mm, 150mm apart.
The Allied 20mm 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 20mm/200 meters/90 degrees 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 18mm homogeneous plate or 16.5mm face-hardened armour at 457 meters/70 degrees. Another US official source gives penetration at 300m as follows: 31-39 mm/90 degrees (depending on the armour hardness), 19mm / 60 degrees and 10mm/30 degrees (presumably for average armour hardness). The Germans surprisingly credited this loading with only 25 mm/100m/90 degrees, although noted that passing through the usual dural skin only reduced this to 21mm. 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/70 degrees, but it seems that it was ballistically unsatisfactory and was not adopted.
The Soviet 20mm ShVak API, which like their 12.7mm had a hard steel core with the incendiary mixture in the tip of the jacket, preformed erratically in the german tests, achieving 24mm/100m/90 degrees, but only 7mm after passing through the dural skin. In the same test, the figures for the 23mm Vya (taken at 300m instead of 100m) were 30mm and 16mm respectively.
One curious aspect of AP performance is that the maximum damage to the aircraft to the aircraft structure was caused when the projectile had only just enough power to penetrate; higher velocities than this merely punched neater holes."
