This fully assembled and NiB coated (5.56/.223) bolt assembly is compatible with most mil spec AR-15, M16, M4 Carriers. EXO Nickel Boron provides permanent dry lubricity, increased wear and corrosion resistance. By reducing friction this bolt assembly will make for easy cleanup and improve the performance of your rifle. Features: Nickel boron treated Extreme durability Dry lubricity Corrosion resistant