AP or "Armor-piercing shell" is a type of ammunition that is designed to penetrate (you guessed it) armour. It is mostly known because of its service in WW1 and WW2 in tanks, warships and anti-tank weaponry. This type of ammunition was slowly abadoned during the 1950s due to the need of ammunition with higher
efficiency and
penetration due to the
introduction of cold-war era MBTs (Main Battle Tanks) and active/passive armour. A few other types of ammunition that were widely used in WW2 are: APCR (Armour Piercing Composit Rigid), HE (High Explosive), HEAT (High Explosive Anti Tank), HESH (High Explosive Squash Head) Modern era ammunition used in the T-62 U-5TS gun: APFSDS (Armour-piercing fin-stabilized discarding sabot), DU (Depleted Uranium), ATGM (Anti Tank Guided Missile)