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The AR-15 family of weapons uses a direct-impingement gas system which does not utilize a gas piston, Figure 2-1. When a round is discharged, the bullet passes the gas port in the barrel where gas is tapped off into the gas tube and redirected back to the bolt carrier key, Figure 2-2. The gas enters the gas carrier key and is channeled into an expansion chamber in the bolt carrier. The gas is trapped between the rear portion of the expansion chamber, and the gas rings towards the rear of the bolt. As the gas expands, it is contained by the bolt gas rings and acts on the bolt carrier, pushing it to the rear, and operating the bolt cam pin in the bolt carrier’s cam surface. As the bolt carrier moves to the rear, the bolt rotates 22.5 degrees clockwise, unlocks, and begins extraction of the expended cartridge case.

VARIANTS

Numerous variants are available throughout the country and world, and nearly all work the same way. The key to keeping this system running reliably, with minimum maintenance, is ensuring that all the critical parts related to extraction and ejection and prone to wear are serviceable. These parts are extractor and spring, ejector and spring, gas rings, and buffer spring. If all these are in good serviceable condition and relatively clean, the AR-15 Carbine is extremely reliable.

NOTE

The buffer in a military configured AR-15/M4 should be marked with an “H” for heavy. This is a heavier buffer and aids in reliability by slowing down the cyclic rate.

Figure 2-1 Direct-Impingement Gas System prior to firing

Figure 2-2 Direct-Impingement Gas System during firing