The Brownells M16 5.56 Complete Bolt-Carrier Group is designed to work easily and reliably in any AR-15 or M16 rifle chambered in 5.56x45mm, 300 AAC Blackout or .223 Remington. The Brownells M16 5.56 Complete Bolt-Carrier Group is constructed with a bolt made from 9310 steel, which is finished with a heat treatment and shot peening. The carrier is fabricated from 8620 hardened steel. Each element of the bolt-carrier group is finished in a black-nitride coating. Magnetic-particle inspected material Extractor includes spring and rubber insert Includes torqued and staked gas key Functions with any cartridge based on the 5.56 NATO cartridge case The Brownel Mfg: Brownells