The Dirty Bird AR-15 Bolt Carrier Group comes fully assembled and ready to be dropped in. The carrier body is machined from 8620 steel, heat treated, & MP inspected. The carrier’s bore is hard chromed for increased corrosion resistance and reliability, per mil-spec. The bolt is machined from Carpenter 158 tool steel, shot peened to relieve stress, individually high-pressure tested and MP inspected. The bolt and carrier are both treated with a Mil-Spec phosphate coating for its durability and corrosion resistance.