armor piercing ammunition is ammunition that is designed to penetrate armor.
Depending on the context, this may refer to armored vehicles or soft body armor. Unless stated otherwise, the context is usually soft body armor.
U.S. Federal law, 18 USC § 921(a)(17), sets a legal definition for armor piercing handgun ammunition based on the construction of the projectile. Ammunition thus constructed may or may not be capable of penetrating soft body armor, but it is nevertheless restricted.
Most rifle ammunition is inherently capable of penetrating most levels of soft body armor, due to its characteristics of velocity and sectional density. Therefore, the term “armor piercing” is not relevant in the context of rifles and soft body armor.