Enhanced Bolt Carrier Group The Enhanced Bolt Carrier Group for AR15/M16 platform weapons was designed to minimize carrier tilt and reduce friction inside the upper receiver by relocating key bearing surfaces. All materials, tolerances, and coatings meet or exceed milspec standards for performance, reliability, and durability. Description Type: 223/5.56 NATO AR15/M16 (full auto) Bolt: Case hardened 9310 steel, shot peened w/ black nitride finish Carrier: Billet machined case hardened 8620 steel w/ black nitride finish Gas Key: 4130 steel, attached using Grade 8 hardware and proper staking techniques Weight: 11.4 oz