What is the function of the bolt carrier group in the M16/M4?

• A. It houses the bolt and firing pin and cycles the action to chamber, extract, and eject rounds.
• B. It feeds ammunition from the magazine.
• C. It directs gas away from the muzzle to reduce recoil.
• D. It houses the rifle's sights.

Answer: (A)

The bolt carrier group is the moving part that handles the firing cycle for the rifle. It contains the bolt and firing pin and, driven by the gas system, performs the sequence that chambers a round, extracts the spent case, and ejects it.

When a shot goes off, high-pressure gas tapers into the gas system and pushes the bolt carrier rearward. That rearward motion unlocks the bolt, the extractor pulls the spent casing from the chamber, and the ejector shoots it out of the rifle. The bolt carrier then returns forward under spring pressure, chambers the next round from the magazine, and locks the bolt back into the barrel so the rifle is ready to fire again. The firing pin is housed in the bolt, so it strikes the cartridge primer when the bolt is in the forward, locked position.

This part doesn’t feed ammunition—the magazine does that. It also isn’t responsible for directing gas away from the muzzle or for housing the rifle’s sights.