FailZero Part Bolt Assembly, Nickel EXO-BOLTASSEMBLY01 This fully assembled and NiB coated (5.56/.223) bolt assembly is compatible with most mil spec AR15" 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.