The .50 Beowulf (12.7x42mmRb) is a rifle cartridge developed by Alexander Arms for use in an AR-15 rifle. The cartridge utilizes a rebated rim, sized to match the rim of the 7.62x39mm and 6.5mm Grendel rounds. It is very similar in dimensions to the .500 S&W Magnum revolver cartridge.
According to the manufacturer, it was descended from the .50 Action Express, a cartridge originally developed for the Magnum Research Desert Eagle pistol, with significant modification to improve functionality and safety in the AR-15 platform. The round is intended to improve terminal ballistics and barrier penetration greatly at short-to-medium range as compared to the standard 5.56mm. 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.
.50 Beowulf cartridges are usually loaded with normal bullet weights between 300 and 400 grains.
Alexander Arms uses gas ports adjusted for a pressure of 15,000 PSI.