Bushmaster's complete upper allows you to add the hard-hitting punch of the 450 Bushmaster to your Bushmaster lower receiver instantly. The AR-type gas operating system drastically reduces recoil for excellent second shot control. This flattop upper includes a 20" heavy barrel with a hard-chrome lined bore and chamber for exceptional durability, and 1:24" twist, optimized for the .450 bullet. The aluminum forearm free-floats the barrel for optimum accuracy. It includes the bolt carrier assembly, and a 5-round 450 Bushmaster magazine.