Which assembly houses the bolt and firing pin in the M16/M4 system?

• A. Bolt carrier group.
• B. Upper receiver.
• C. Gas tube.
• D. Lower receiver.

Answer: (A)

The moving assembly that carries both the bolt and the firing pin is the bolt carrier group. In the AR-15/M16/M4 platform, the bolt and its firing pin are housed together within this group, which travels back and forth inside the upper receiver as the gun cycles. The firing pin is part of the bolt itself, extending through the bolt face to strike the cartridge primer, while the bolt carrier group as a whole includes the bolt, firing pin, cam pin, and other related pieces working as a unit. The upper receiver contains the BCG as it cycles, the lower receiver holds the trigger and hammer, and the gas tube is part of the gas system—not where the bolt or firing pin reside.