When it comes to the hard hitting and high pressue 450, dont settle for a mil-spec upper. Trident Upper receivers are built around the 450 cartridge. The same Upper receiver and assembly found on our rifles can now be purchased individually. 450 Bushmaster uppers come in two ways; Standard is a 18" ported barrel with a Hogue rifle length free-float tube. One 5rnd magazine is included. Or, You can have a complete upper custom built to your specs!