Ar15 bolt carrier goup is made of Mil-spec Carpenter 158 steel, Shot peened, High pressure tested and Mag Particle Inspected.   Gas key is machined out of bar stock, chrome lined phosphate coated, and secured with grade 8 fasteners and staked per mil-spec. AR 15 Bolt carrier assembly is phosphated outside and chrome lined inside, our M16 bolt carrier assembly comes with different caliber bolts depending on specific caliber built.