In short, the .50 Beowulf ammo is an elephantine round designed to increase stopping power of an AR-15-platform rifle by a factor of 10 billion or so. The cartridge is just short enough to fit in a standard AR magazine, although for obvious reasons (it’s fat) it won’t double stack like .223 Rem./5.56 NATO cartridges. Developed by Alexander Arms, this cartridge is a tapered, straight-wall case topped with, you guessed it, a .50-caliber projectile. 50 Beowulf ammo
So that it can use a 7.62×39 mm bolt face, the case rim is rebated, or smaller than the case body. This not only allows use of a “more standard” bolt, but it also allows that bolt to have enough metal on it to withstand pressure over time and use.
As for the launcher part, Alexander Arms offers both complete upper receivers and fully-built rifles. If you go the upper receiver route on a lower of your own, you’ll need the installed barrel with the much bigger hole.
What’s not as obvious is that the Alexander Arms upper receivers include a bolt carrier and bolt compatible with that 7.62×39 mm rebated cartridge rim. What’s even less obvious is that the Alexander upper has an enlarged ejection port. Those cases are on the chubby side and need a bit more elbow room to escape.
Most of the current upper-receiver models include 16.5-inch barrels. However, if you’re in the door-kicking business, you might be interested in the 12-inch barrel version. Unless you configure the world’s most unwieldy AR pistol, you’ll need to obtain an ATF short barrel rifle tax stamp for that one. The pre-built rifles all sport 16.5-inch barrels and come in a variety of fixed and adjustable stock configurations. All of them are flattop receivers, but you can certainly stick that classic AR carry handle on if want to be a purist tactical poet.