This drop in ready upper receiver assembly features a forged aluminum Mil-Sport AR-15 pattern upper receiver with a 16 barrel with 1:24 button cut rifling and black nitride finish chambered for 450 Bushmaster. Completed with a 15 aluminum free floated key-mod rail system over a carbine length low profile DGIS gas system A2 style compensator and black nitride finished bolt carrier group. Fits to any AR-15 pattern lower receiver. - 450 Bushmaster Caliber - 16 Black Nitride Barrel - 7075-T6 Forged Aluminum Upper Receiver - 15 Free Float Key-Mod Rail System - Low Profile Carbine Length Gas System (DGIS) - A2 Style Compensator - Black Nitride Finish Bolt Carrier Group