Centurion Arms Bolt Carrier GroupCenturion Arms C4 Bolt Carrier Group was created with HP/MPI tested bolts and marked to give you the ultimate confidence in your weapon system. This full auto carrier is heat treated and precision machined. The gas key is staked on both sides of both bolts. Specs: Mil-spec Carpenter No. 158 HP/MPI Inspected bolt Shot peened bolt Chrome lined carrier (auto) Chrome lined gas key Gas key hardened to mil spec Grade 8 fasteners Staked gas key Tool steel extractor