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.Features :.450 Bushmaster16 Inch Chrome Moly, 1:24 TwistRRA Operator Brake, 5/8-32 ThreadLow Profile Gas Block, Mid-Length Gas SystemRRA 13 Inch Extended Lightweight Free Float RailBolt Carrier Group, Charging Handle Assembly(1) Magazine, (5) Rounds