Armor piercing round

An armor-piercing shell,AP for short, is a type of ammunition designed to penetrate armor. From the 1860s to 1950s, a major application of armor-piercing projectiles was to defeat the thick armor carried on many warships. From the 1920s onwards, armor-piercing weapons were required for anti-tank missions. AP rounds smaller than 20 mm are typically known as "armor-piercing ammunition", and are intended for lightly-armored targets such as body armor, bulletproof glass and light armored vehicles. The classic AP shell is now little used in naval warfare, as modern warships have little or no armor protection, and newer technologies have displaced the classic AP design in the anti-tank role.

An armor-piercing shell must withstand the shock of punching through armor plating. Shells designed for this purpose have a greatly strengthened body with a specially hardened and shaped nose. One common addition to later AP shells is the use of a softer ring or cap of metal on the nose known as a penetrating cap, which both lowers the initial shock of impact to prevent the rigid shell from shattering, as well as aiding the contact between the target armor and the nose of the penetrator to prevent the shell from bouncing off in glancing shots. Ideally, these caps have a blunt profile, which led to the use of a thin aerodynamic cap to improve long-range ballistics. AP shells may contain little or no explosive, in this case known as a "bursting charge". Some smaller-caliber AP shells have an inert filling or incendiary charge in place of the bursting charge.

As tank armor improved, especially during World War II, AP designs were introduced that used a smaller penetrating body within a larger shell. These lightweight shells fired at very high muzzle velocity and retained that speed and the associated penetrating power over longer distances. In modern designs the penetrator no longer looks like a classic artillery shell design, but is instead a long rod of dense material like tungsten or depleted uranium (DU) that further improves the terminal ballistics. Whether these designs are considered to be AP rounds depends on the definition and may be included or excluded from reference to reference. Other anti-armor technologies, like the high explosive anti-tank (HEAT) round, are not considered to be AP rounds in most definitions.

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