AR-15 parts and names

Last reviewed October 26, 2026 · 3 sources

Short answer: An AR-15 splits into two main assemblies, an upper and a lower. Federal regulations name the lower receiver, which houses the trigger mechanism and hammer, as the legal “receiver” for AR-15/M-16 variants. [27 CFR § 478.12]

Diagram

Lower receiver group

Upper receiver group

Bolt carrier group (BCG)

Stoner’s patent describes the bolt and bolt carrier at the center of the operating system. The bolt locks the breech and acts as a stationary piston. The carrier acts as a movable cylinder, and it locks and unlocks the bolt by rotating it. [US Pat. 2,951,424]

Gas system, barrel, and handguard

The legal “frame or receiver”

Under 27 CFR § 478.12, the “receiver” of a rifle is “the part … that provides housing or a structure for the primary component designed to block or seal the breech prior to initiation of the firing sequence.” [27 CFR § 478.12, (a)(2)] The same section lists AR-15/M-16 variant firearms as an example and identifies their receiver as the lower receiver. [27 CFR § 478.12] The federal definition of “firearm” includes the frame or receiver. [18 U.S.C. § 921, (a)(3)]