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The .450 Bushmaster was designed by the Hornady Manufacturing Company for Bushmaster Firearms International when the latter wanted a .45 caliber cartridge that would work through the short AR-15 action. The result of this collaboration is the .450 Bushmaster.
The Bushmaster is based on a sawed-off .284 Winchester case. It is a rimless, straight wall case with a rebated .473 inch diameter rim and a .500 inch diameter head. The case is 1.70 inches long. It uses .452 inch diameter bullets, as used in .45 caliber handguns, not standard .458 inch diameter rifle bullets. The loaded overall cartridge length is 2.250 inches and the case takes small rifle primers.
The .450 Bushmaster's MAP is 38,500 psi, considerably below that of the .223 Remington. However, the Bushmaster has much greater back thrust against the bolt face, so be careful not to exceed the SAAMI pressure limit.
Lacking a rim, shoulder or belt on which to headspace, the case must headspace on its mouth, like a typical autoloading pistol cartridge (.45 ACP, etc.). This prevents the use of a roll crimp, as used by most big bore rifle cartridges, to keep the bullet in place during recoil.

450 BushMaster - This is a straight-walled rifle cartridge designed for the Lighter Rifle and AR-15 platform which also meets the Michigan requirements. The .450 Bushmaster thumper round launches .452-cal. bullets weighing 250 gr. or 260 gr. from a 1.70" long cartridge case at velocities over 2000 fps. The resulting round boasts performance on par with the .45-70 Gov't but in a more compact configuration.

450 Bushmaster