BCM® (MPI) complete bolt carrier group is fully assembled. (BCG) Bolt carrier group (auto version) for your AR15, M16, or M4. Parkerized exterior and chrome lined inside carrier. Machined to USGI specifications. This M16/M4 carrier includes the proper Mil-Spec gas key. Gas key is chrome lined and heat treated per GI specifications. Gas key is secured to carrier via USGI domestic mfg Grade 8 fasteners and properly staked per Mil-Specs. These properly hardened parts insure a strong staking and proper gas system functioning.