How an AR-15 works

Last reviewed October 26, 2026 · 6 sources

Short answer: A civilian AR-15 is a semi-automatic rifle. Federal law defines a “semiautomatic rifle” as “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, (a)(29)] The AR-15 gets that energy from propellant gas, using the bolt-and-carrier system described in Eugene Stoner’s U.S. Patent 2,951,424. [US Pat. 2,951,424]

The cycle, step by step

Stoner’s patent illustrates the gas system at successive stages. It shows the moment just after firing, the point just before the cartridge case is pulled from the chamber, and the point where the case has been removed and is about to be ejected. [US Pat. 2,951,424, Figs. 1, 4–6] In plain terms:

1. Fire. The trigger releases the hammer, and the cartridge fires.
2. Gas tap. Once the bullet passes a port in the barrel, some of the propellant gas flows back through a passage to the bolt carrier. [US Pat. 2,951,424]
3. Unlock. The bolt carrier’s job is “to lock and unlock the bolt by rotating it and to carry it back and forth in the receiver.” [US Pat. 2,951,424]
4. Extract and eject. The bolt pulls the empty case from the chamber, and the patent describes a conventional ejector “for kicking the shell out of the gun.” The case exits through a slot in the receiver. [US Pat. 2,951,424]
5. Feed, chamber, lock. The bolt and carrier return forward, strip a fresh cartridge from the magazine, chamber it, and lock the bolt again. The rifle is then ready for the next separate trigger pull.

Stoner’s gas system (“direct impingement”)

Stoner’s patent describes a bolt with a “double function.” Its primary job is to lock the breech against the pressure of firing. Its secondary job is to “act as a stationary piston.” The bolt carrier, in turn, acts “as a movable cylinder.” [US Pat. 2,951,424] This arrangement is commonly called direct impingement, because gas acts inside the carrier rather than on a separate piston and operating rod.

Upper and lower receivers

For AR-15/M-16 variant firearms, federal regulations identify the receiver as “the lower part of the weapon that provides housing for the trigger mechanism and hammer (i.e., lower receiver).” [27 CFR § 478.12] Because the federal definition of “firearm” includes “the frame or receiver of any such weapon,” [18 U.S.C. § 921, (a)(3)] the lower receiver is what the law treats as the firearm. The upper assembly holds the barrel and bolt carrier group. See Parts & names.

Semi-automatic vs automatic

Federal law draws the line by what one function of the trigger does:

• Semiautomatic rifle: fires one cartridge for each separate pull of the trigger. [18 U.S.C. § 921, (a)(29)]
• 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.” [26 U.S.C. § 5845, (b)]

Federal law generally makes it unlawful to transfer or possess a machinegun, with narrow exceptions for government use and for machineguns lawfully possessed before that provision took effect. [18 U.S.C. § 922, (o)] In 2024, the Supreme Court held that a semi-automatic rifle fitted with a bump stock is not a “machinegun” under § 5845(b), because the shooter still has to release and reset the trigger between shots. [Cargill (2024)]

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