From The Manufacturer/Wikipedia/People Smarter Than Us:
The Bushmaster .450 – big bore power in an “AR” platform… This powerful cartridge – developed in affiliation with Hornady – is tamed by the AR type gas operating system so that recoil is reduced to a heavy thump, and second shot control is retained. The Cartridge is adapted from a 6.5mm – .284 case with necking cut off to accommodate the 250 Grain Hornady SST Flex Tip .450 caliber bullet.