An Armour piercing shell is one designed to penetrate armour. In naval warfare and older anti-tank shells, the shell had to withstand the shock of punching through armour plate. Shells designed for this purpose had a greatly strengthened case with a specially hardened and shaped nose, and a much smaller bursting charge. Some smaller calibre AP shells had no bursting charge at all. Plain AP shell is now very rarely seen except in naval usage, and is less common even there
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On the introduction of iron ships in the late 19th century it was found that the ordinary cast-iron projectile readily pierced the thin plating, and in order to protect the vital parts of the vessel wrought-iron armour of considerable thickness was placed on the sides. It then became necessary to produce a projectile which would pierce this armour. This was effected by Sir W. Palliser, who invented a method of hardening the head of the pointed cast-iron shot. By casting the projectile point downwards and forming the head in an iron mould, the hot metal was suddenly chilled and became intensely hard, while the remainder of the mould being formed of sand allowed the metal to cool slowly and the body of the shot to be made tough.
These shot proved very effective against wrought-iron armour, but were not serviceable against compound and steel armour. A new departure had, therefore, to be made, and forged steel shot with points hardened by water, took the place of the Palliser shot. At first these forged steel shot were made of ordinary carbon steel, but as armour improved in quality the projectiles followed suit, and, for the attack of the latest type of cemented steel armour, the projectile is formed of steel -- either forged or cast -- containing both nickel and chromium. Tungsten steel were also been used with success.
These pre-WWI Armour-piercing shot or shell were generally cast from a special mixture of chrome steel melted in pots. They were afterwards forged into shape and then thoroughly annealed, the core bored and the exterior turned up in the lathe. The shells were finished in a similar manner to others described below. The final or tempering treatment was very important, and was a closely gaurded secret. It consisted of hardening the head of the projectile and tempering it in a special manner, the rear portion being reduced in hardness so as to render it tough. The cavity of these projectiles were capable of receiving a small bursting charge of about 2% of the weight of the complete projectile, and when this is used the projectile is called an armour-piercing shell. The shell, whether fuzed or unfuzed, will burst on striking a medium thickness of armour. Armour-piercing shells of this period had a bursting charge of about 3% of the weight of the complete projectile, are were often fitted with a soft
steel cap for the perforation of hard steel armour.
Even with these improvements the projectile was not,
with a reasonable velocity, able to pierce one
calibre in thickness of cemented steel armour. History
