The .50 Beowulf is a rifle cartridge developed by Bill Alexander of Alexander Arms for use in a modified AR-15 rifle. The cartridge utilizes a rebated rim, sized to match the rim of the 7.62×39mm and 6.5mm Grendel rounds. The case body is very similar in dimensions to the .500 S&W Magnum revolver cartridge, being slightly longer and fully tapered for automatic feeding in the weapon.

The round is intended to improve stopping power greatly at short- to medium-range as compared to the standard 5.56×45mm NATO round. One of its advertised uses is at vehicle checkpoints, since the heavy bullet's flight path is not easily deflected by auto glass or standard vehicle body panels.

With normal bullet weights between 300 and 400 grains (19 and 26 g), overall cartridge length shorter than that of an AR-15 magazine well, and holding to pressures of 33,000 psi limited by the AR bolt strength system, the .50 Beowulf is best described as a low-velocity, heavy caliber, making its ballistics roughly equivalent to those of early .45-70 Government rounds rather than the higher pressure rounds tolerated by modern lever-action rifles such as the Marlin Model 1895. Adaptability is limited due to its use of the 7.62×39mm bolt face.