Wilson Combat Bolt Carrier Group Assembly, 223 Remington/556NATO, Magnesium Phosphate Finish, Black, Fits AR-15 TR-BCA Manufactured by their finest domestic Mil-Spec contractors these are among the best 5.56 AR rifle bolts on the market and the same bolts they use as the basis of their renowned custom rifles! Their bolt is precision machined from Carpenter 158" shot peened for increased strength and includes a tool steel extractor and ejector" appropriate extractor spring" and randomly batch HPT (High Pressure Tested). The extractor is 4140 gun quality" annealed" cold drawn steel per the Mil-Spec. Parkerized externally and internally chrome lined the 8620 steel carrier includes a proper machined Mil-Spec gas key that is chrome lined" phosphated and heat treated per GI specifications. It is secured to the carrier via Grade 8 fasteners and properly staked in their shop during final inspection. These precisely machined" finished and hardened parts will ensure long term proper gas system functioning and reliability. All experienced AR shooters know that the best spare parts kit for an AR-15 rifle is a spare complete bolt carrier assembly!