An armor-piercing or “AP” round is a type of ammunition used to penetrate armored objects. From the 1860s to 1950s, a major usage of armor-piercing projectiles was to penetrate the thick armor carried on many warships. From the 1920s onwards, armor-piercing weapons were required for anti-tank missions. “Shots” and “rounds” 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-material round.

An armor-piercing round must withstand the shock of punching through armor plating. Rounds designed for this purpose have a greatly strengthened jacked with a specially hardened and shaped nose, and a often a hardened core. Some smaller-caliber AP rounds have an inert filling, or incendiary charge in place of the HE bursting charge. The AP round 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.

Armor-piercing solid shot for cannons may be simple, or composite, solid projectiles but tend to also combine some form of incendiary with that of armor-penetration. The incendiary compound is normally contained between the cap and penetrating nose, within a hollow at the rear, or a combination of both. If the projectile also uses a tracer, the rear cavity is often used to house the tracer compound. For larger-caliber projectiles, the tracer may instead be contained within an extension of the rear sealing plug. Common abbreviations for solid (non-composite/hardcore) cannon-fired shot are; AP, AP-T, API and API-T; where “T” stands for “tracer” and “I” for “incendiary”. More complex, composite projectiles containing explosives and other ballistic devices tend to be referred to as armor-piercing rounds.

Armor-piercing rounds as dedicated anti-tank projectiles are common in over 50mm caliber artillery but the tendency is to use semi-armor-piercing and high-explosive (SAPHE) rounds, which have less anti-armor capability, but far greater anti-material/personnel effects. The modern SAPHE projectiles still have a ballistic cap, hardened body and base fuze, but tend to have a far thinner body material and higher explosive content 4–15%. Common abbreviations for modern AP and SAP rounds are: HEI(BF), SAPHE, SAPHEI, and SAPHEI-T.

Small Arms Armor-piercing rifle and pistol cartridges are usually built around a penetrator of hardened steel, tungsten, or tungsten carbide, and such cartridges are often called ‘hard-core bullets’. Aircraft and tank rounds sometimes use a core of depleted uranium. The penetrator is a pointed mass of high-density material that is designed to retain its shape and carry the maximum possible amount of energy as deeply as possible into the target. Depleted-uranium penetrators have the advantage of being pyrophoric and self-sharpening on impact, resulting in intense heat and energy focused on a minimal area of the target’s armor. Some rounds also use explosive or incendiary tips to aid in the penetration of thicker armor. High Explosive Incendiary/Armor Piercing Ammunition combines a tungsten carbide penetrator with an incendiary and explosive tip.

Rifle armor-piercing ammunition generally carries its hardened penetrator within a copper or cupronickel jacket, like the jacket which would surround lead in a conventional projectiles. Upon impact on a hard target, the copper case is destroyed, but the penetrator continues its motion and penetrates the target. Armor-piercing ammunition for pistols has also been developed and uses a design like the rifle ammunition. Some small ammunition, such as the FN 5.7mm round, is inherently capable at piercing armor, being of a small caliber and very high velocity.

The entire projectile is not normally made of the same material as the penetrator because the physical characteristics that make a good penetrator (i.e. extremely tough, hard metal) make the material equally harmful to the barrel of the gun firing the cartridge.