AR-15 glossary

Last reviewed October 26, 2026 · 7 sources

These definitions come from the source listed with each one. Where a term has a legal meaning, we use the statute’s or regulation’s wording. Some are lightly condensed, and the full text is one click away.

Bolt and bolt carrier

In Stoner’s gas system, the bolt locks the breech and also acts as a stationary piston, while the bolt carrier acts as a movable cylinder that locks and unlocks the bolt by rotating it and carries it back and forth in the receiver. [US Pat. 2,951,424]

Bump stock

An accessory designed to make bump firing easier. The Supreme Court held in 2024 that a semi-automatic rifle with a bump stock is not a “machinegun” under 26 U.S.C. § 5845(b). [Cargill (2024)]

Firearm (18 U.S.C. § 921)

Includes any weapon that will or is designed to or may readily be converted to expel a projectile by the action of an explosive, and the frame or receiver of any such weapon (among other items). [18 U.S.C. § 921]

Lower receiver

For AR-15/M-16 variant firearms, the part ATF regulations identify as the receiver: the lower part of the weapon that houses the trigger mechanism and hammer. [27 CFR § 478.12]

Machinegun

Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger (plus its frame or receiver and certain conversion parts). [26 U.S.C. § 5845]

Maximum Average Pressure (MAP)

SAAMI’s recommended maximum pressure level for loading commercial sporting ammunition. [SAAMI Z299.4-2025]

Receiver (rifle)

The part of a rifle that provides housing or a structure for the primary component designed to block or seal the breech prior to initiation of the firing sequence (bolt, breechblock, or equivalent). [27 CFR § 478.12]

Rifle (NFA)

A weapon designed, made, and intended to be fired from the shoulder and to fire only a single projectile through a rifled bore for each single pull of the trigger, using the energy of the explosive in a fixed cartridge. [26 U.S.C. § 5845]

Semiautomatic assault weapon (1994–2004 federal term)

A term defined by the 1994 federal crime law through named firearms (including the Colt AR-15) and a features test; the subtitle was repealed effective ten years after enactment. [Pub. L. 103-322]

Semiautomatic rifle

Any repeating rifle which utilizes a portion of the energy of a firing cartridge to extract the fired cartridge case and chamber the next round, and which requires a separate pull of the trigger to fire each cartridge. [18 U.S.C. § 921]