 The .450 Bushmaster is descended from the "Thumper" concept popularized by 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.Inspired by this, LeGendre developed his .45 Professional cartridge, and later built and delivered an AR-15 in .45 Professional to Cooper.