During WW II, a phenomenon known as shatter gap resulted in hits with too much penetration failing to defeat the armor.
The British noted this oddity in Libya and other North African areas, where rounds that could penetrate beyond 1000 yards would fail at shorter ranges, or hits would fail at short range and then start to penetrate further out.
The theory on shatter gap is that when hits penetrate on half the hits at a given velocity (the basis for most penetration data), there are certain impact forces on the projectile nose. If the velocity is increased and the armor thickness is held constant, the round moves armor out of the way faster, which leads to increased inertial forces on the ammo nose.
If the projectile nose is too soft, such that it absorbs much of the impact energy, the nose can shatter and break up. U.S. and Russian ammunition fell into the shatter gap nose hardness range (less than 59 Rockwell C). While British ammunition was harder than the threshold, some characteristic of the projectiles made it vulnerable to shatter gap.
With regard to Tiger armor, shatter gap normally occurs when the armor thickness is close to, equal to or thicker than the projectile diameter. U.S. 76mm APCBC hits on Tiger armor would fall into this category.
If 76mm APCBC hit the Tiger driver plate at 12° side angle, the resultant resistance would equal 109mm at 0°. With shatter gap, rounds fail when they have 1.05 to 1.25 times the armor resistance, which would result in M10 failures from point blank to 550 meters range, and then penetrate from 550m to 750m.
On M10 hits against the Tiger side armor at 30° side angle, the resistance would equal 103mm at 0°, and M10 hits would be expected to fail from point blank to 800m, and then penetrate from 800m to 1000m.
U.S. Navy tests during WW II against 3" armor at 30°, using 76mm APCBC, resulted in 50% penetration at about 2069 fps impact, and then the hits failed from 2073 fps through 2376 fps.
Firing tests with 75mm APCBC did not appear to result in shatter gap failures, suggesting that impact velocities above 2000 fps would be required for nose failure.
Prior to Normandy, the Americans calculated that their 76mm gun would be sufficient to stop Panthers and Tigers, since the 100mm frontal armor on those panzers could theoretically be penetrated to 1250m by M10's and 76mm armed Shermans. Shatter gap may be responsible, in part, for the sorry showing of those guns in France against heavy German armor.
