Rock River Arms Tactical Carbine, Complete Upper Receiver, 458 Socom, 16 Barrel, RRA Operator Muzzle Brake, RRA 15 Free Float M-LOK Handguard, Anodized Finish, Black, Fits AR-15, Includes Bolt Carrier Group and Charging Handle Assembly SOC0564 Rock River Arms complete Tactical Carbine upper is an A4 upper chambered in .458 SOCOM with a 16" chrome moly bull barrel and 1:14 twist. Additionally the upper comes with a complete bolt carrier group" charging handle assembly" 15" RRA lightweight M-Lok compatible free float rail and a RRA Operator brake with a a 5/8-32 thread pitch.