The Micro-Best Bolt Carrier Group (BCG) (auto version) for 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 ensure a strong staking and proper gas system functioning. The bolt assembly is machined from the correct Mil-Spec Carpenter No. 158® steel, shot peened for increased strength, includes tool steel machined extractor and ejector, and is Individual HPT (High Pressure Tested) and MPI (Magnetic Particle Inspected). These Bolt groups are manufactured by Micro-Best, the oldest and largest OEM supplier of Mil-Spec BCG for many of the Govt. contracts.