Heavy bolt with 15 parts in this assembly with 11 improvements. 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 (tested full-auto 2/10/13). Includes Colt M16 firing pin upgrade. Properly staked gas key prevents gas leakage that can lead to short stroking. Available in .223/5.56mm, .300 Blackout ONLY... This assembly is made to last and to perform in the worst conditions. will perform "dry" without damage. Staked screws, parkerized exterior with hardened interior for bolt and cam pin.