Toolcraft Nickel Boron 158 Bolt Carrier Group - MPI Bolt - M16 CarrierThis Toolcraft Nickel Boron 158 BCG comes with a lifetime warranty and is perfect for any AR15, M4 or M16 Build. Toolcraft carriers and bolts are build to military specifications, also known as "Mil-Spec" and they are known throughout the industry as some of the best. Toolcraft Bolt Carrier Groups come with a lifetime warranty! Mil-spec carpenter No. 158® steel (bolt)8620 Tool Steel (Carrier)Mil-Spec Extractor Spring, black extractor insert, Viton O-RingTool Steel ExtractorShrouded Firing PinMil-Spec Gas Key (4130 Steel) attached with Grade 8 hardware and Properly stakedCaliber: .223 / 5.56 / 300 Blackout Bolt is shot Peened and MPIToolcraft has all BCG compoonents pre polished before being plated by UCT and then post polished for an incredible shine.