Anderson 20” Bull Barrel Complete Upper With EXT Forearm, 20” HBAR Barrel, (heavy barrel) 1-9 twist, gun-barrel-certified 4150 Chrome Moly Steel with M4 Barrel Extension. Chambered in 5.56/ .223, with gas hole drilled and Parkerized, Rifle Length Gas System, .936 gas block diameter,. Anderson EXT Hand Guard Rifle Length, M16 Carrier is Chrome Lined, made from 8620 steel, Bolt is made from 9310 steel and coated per mil spec. MPI Tested. The 223 Bolt and Bolt Carrier Assembly comes complete with Cam Pin, Firing Pin and Firing Pin Retainer and Ambi Charging Handle.