Gas-operated firearms, including various assault rifles, require a bolt carrier group (BCG) to complete the cycle when a round is fired. A complete BCG pulls a round from the magazine and loads it into the chamber before locking back into place. A popular type is the 5.56 bolt carrier group, which also works as a 5.56 .223 bolt carrier group. However, some rifle owners do not understand that many 5.56/.223 BCGs can be used as a 300 BLK BCG.
All of the bolt carrier groups from Right to Bear Arms and Supply include special features that provide the best bang for your buck. Many of them use a Carpenter 158 bolt, but some have a 9310 bolt. Models from Spike's Tactical and others are high pressure tested – HPT – and magnetic particle inspected – MPI. In addition, the Fail Zero BCG uses a full auto carrier.