The .450 Bushmaster is designed to be used in standard M16s and AR-15s, using modified magazines and upper receiver assemblies.

The .450 Bushmaster is descended from the Thumper concept popularized by the gun writer Jeff Cooper.

Cooper was dissatisfied with the small-diameter 5.56×45mm NATO (.223 Remington) of the AR-15, and envisioned a need for a large bore (.44 cal or greater) cartridge in a semi-automatic rifle to provide one-shot kills on big-game animals at 250 yards.

the ammunition manufacturer Hornady to produce the .45 Professional cartridge for this project, but Hornady wanted to shorten the cartridge case and overall length to accommodate their 0.452 in.

bullets because the lower impact velocities and energies would not adequately expand the heavier jacketed .458 in.