At varying points in the last decade, several enterprising souls looked into an AR magazine and thought to fill it up completely with one massive cartridge instead of a couple little ones. The resulting cartridges—the .450 Bushmaster, .458 SOCOM, and .50 Beowulf, among others—are big league heavy hitters.
They all use a rebated rim to fit inside a standard AR bolt head and generally run at pressures well below that of the 5.56mm. The only real concern with fitting cartridges of this size in an AR resides in the bolt thrust and making sure the gas port is both the right size and sits in the right place for reliable function. All three rounds obviously produce substantially more recoil than 5.56mm rifles, but the rifle seems up to the task. (Scope mounts are another matter.)
Rifle manufacturers seem to have divvied up the calibers between themselves, with Rock River Arms building .458 SOCOM rifles, Bushmaster producing the .450 Bushmaster, and Alexander Arms building rifles chambered for the cartridge it invented. Ammunition is a little hard to find and, in some cases, expensive, but available. Reloaders usually prefer the SOCOM over the Bushmaster and Beowulf, because the wider selection of available bullets.
The .458 and .50 were envisioned as interdiction and barrier-beating rounds for stopping cars at roadblocks and ventilating two-legged predators hiding behind cinderblock walls and the like. It seems most shooters use these big-boress to turn mobile pork into stationary pork. The rounds are also ideal for close-range bear hunting or hitting anything with bullets weighing in excess of 250 grains and moving over 2,000 fps. Most Beowulf loads produce over 2,900 foot-pounds (ft-lbs) of muzzle energy.
Imagine the possibilities of combining one of these big-bore cartridges loaded with good, solid bullets and a lightweight AR carbine fed from a high-capacity detachable magazine. Pair the rifle with a weapons light and a 1X-optic like an Aimpoint, Trijicon, or EOTech, and you might have the perfect guide’s gun for backing up hunters or tracking wounded bears in Alaska. It weighs less, provides more firepower, and permits faster follow-up shots than most other rifle actions.