Head Down M16 Bolt Carrier Group Complete, mil-spec assembly with bolt, bolt carrier, and firing pin ready to drop into the rifle or carbine upper of your choice. Features a shot-peened bolt for increased surface strength and durability, and an extractor booster to ensure reliable cycling during rapid semi-auto or full-auto fire. Each assembly is magnetic particle inspection tested to ensure it is free ofÂ imperfections that can lead to premature failures. Properly staked gas key prevents gas leakage that can lead to short stroking. M16-style carrier on the black phosphate model has full-length shroud for tripping auto sear, so this carrier may be installed on full-auto law enforcement and military rifles. Chrome Bolt Carrier Group features a Semi-Auto Bolt Carrier. Carrier is chrome lined for smooth cycling and resistance to wear from hot combustion gases for years of dependable service.