SI Defense AR-15 bolt carrier groups are 100% mil-spec, fully assembled and ready to be dropped in. The carrier body is machined from 8620 steel, heat treated, MP inspected and then Mil-DTL-16232G manganese phosphated. The carrier's bore is hard chromed for increased corrosion resistance and reliability. The bolt is machined from Carpenter 158 steel, shot peened, MP inspected and manganese phosphated per Mil-DTL-16232G. SI Defense Bolt Carrier Groups feature a gas key tube that is hard chromed and properly torqued and staked. The SI Defense Bolt Carrier Assembly is tough as they come.
Note: The only difference between the Mil-Spec and Commercial carrier assemblies is the size of the cut on the bottom. The Mil-Spec is a "full-auto" carrier while the Commercial is a "semi-auto" otherwise they are identical.