Rock River Arms Tactical Carbine, Complete Upper Receiver, 450 Bushmaster, 16 Barrel, Mid-Length Gas System, RRA Operator Muzzle Brake, RRA 13 Free Float M-LOK Handguard, Anodized Finish, Black, Fits AR-15, 5 Rounds, 1 Magazine, Includes Bolt Carrier Group and Charging Handle Assembly 450B0592 Rock River Arms A4 mid-length upper comes chambered in .450 Bushmaster with a 16" chrome moly 1/24 twist barrel. Additionally this upper includes a RRA 13" M-Lok compatible lightweight free float rail and a RRA Operator Brake (5/8-32 thread). This is a complete upper with bolt carrier group" charging handle assembly and a 5 round magazine.