An armor-piercing (AP) shell 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. "Shots" and "shells" are typically artillery projectiles, and are used to defeat heavily armored targets such as tanks, bunkers and armored warships. Projectiles smaller than 20mm are typically known as "armor-piercing ammunition", and are intended for lightly-armored targets such as body armor, bulletproof glass and other protection, or for use as an anti-matériel round. An armor-piercing shell must withstand the shock of punching through armor plating. Shells designed for this purpose have a greatly strengthened case with a specially hardened and shaped nose, and a much smaller bursting charge. Some smaller-caliber AP shells have an inert filling, or incendiary charge in place of the HE bursting charge. The AP shell is now little used in naval warfare, as modern warships have little or no armor protection, but it remains the preferred round in anti-tank warfare, as it has a greater "first-hit kill" probability than a high explosive anti-tank (HEAT) round, especially against a target with composite armor, and because of higher muzzle velocity, is also more accurate than a HEAT round.

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