Bolt Carrier
The assembly that houses the bolt and cycles the action of a semi-auto rifle.
Also: BCG, bolt carrier group
The bolt carrier holds the bolt and translates gas/recoil energy into the cycle: unlock, extract, eject, cock the hammer, feed a fresh round, lock. On AR-pattern rifles, the bolt carrier group (BCG) is one of the most-discussed wear items — it sees enormous heat and friction.