Armor-piercing Shot and Shell - Second World War

Second World War

During World War II, projectiles used highly alloyed steels containing nickel-chromium-molybdenum, although in Germany, this had to be changed to a silicon-manganese-chromium-based alloy when those grades became scarce. The latter alloy, although able to be hardened to the same level, was more brittle and had a tendency to shatter on striking highly sloped armor. The shattered shot lowered penetration, or resulted in total penetration failure; for armor-piercing high-explosive (APHE) projectiles, this could result in premature detonation of the HE filling. Highly advanced and precise methods of differentially hardening the projectile were developed during this period, especially by the German armament industry. The resulting projectiles gradually change from high hardness (low toughness) at the head to high toughness (low hardness) at the rear and were much less likely to fail on impact.

APHE shells for tank guns, although used by most forces of this period, were not used by the British. The only British APHE projectile was the Shell AP, Mk1 for the 2 pdr anti-tank gun and this was dropped as it was found that the fuze tended to separate from the body during penetration. Even when the fuze didn’t separate and the system functioned correctly, damage to the interior was little different from the solid shot, and so did not warrant the additional time and cost of producing a shell version. APHE projectiles of this period used a bursting charge of about 1–3% of the weight of the complete projectile, the filling detonated by a rear mounted delay fuze. The explosive used in APHE projectiles needs to be highly insensitive to shock to prevent premature detonation. The US forces normally used the explosive Explosive D, otherwise known as ammonium picrate, for this purpose. Other combatant forces of the period used various explosives, suitability desensitized (usually by the use of waxes mixed with the explosive).

Due to the increase in armor thickness during the conflict, the projectiles’ impact velocity had to be increased to ensure perforation. At these higher velocities, the hardened tip of the shot or shell has to be protected from the initial impact shock, or risk shattering. To raise the impact velocity and stop the shattering, they were initially fitted with soft steel penetrating caps. The best performance penetrating caps were not very aerodynamic, so an additional ballistic cap was later fitted to reduce drag. The resulting projectile types were named armor-piercing capped (APC) and armor-piercing capped ballistic capped (APCBC).

Early WWII-era uncapped AP projectiles fired from high-velocity guns were able to penetrate about twice their caliber at close range (100 m). At longer ranges (500-1,000 m), this dropped 1.5–1.1 calibers due to the poor ballistic shape and higher drag of the smaller-diameter early projectiles. Later in the conflict, APCBC fired at close range (100 m) from large-caliber, high-velocity guns (75–128 mm) were able to penetrate a much greater thickness of armor in relation to their caliber (2.5 times) and also a greater thickness (2–1.75 times) at longer ranges (1,500–2,000 m).