The .50 Beowulf [12.7 x 42 mm Rb] is a rifle cartridge developed by Alexander Arms for use in a modified AR-15 rifle. The cartridge utilizes a rebated rim, sized to match the rim of the 7.62x39mm and 6.5 mm 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 Israel Military Industries Desert Eagle pistol, with significant modification to improve functionality and safety in the AR-15 platform
.50 Beowulf 334 gr. hollowpoint shot from 16" Barrel Beowulf rifle at 36 yards penetrated one quarter inch (1/4") thick steel plate.