High Quality Content by WIKIPEDIA articles! .303 British, or 7.7x56mmR, is a .311 inch calibre rifle and machine gun cartridge first developed in Britain as a blackpowder round put into service in December 1888 for the Lee-Metford rifle, later adapted to use smokeless powders. It was the standard British and Commonwealth military cartridge from 1889 until the 1950s when it was replaced by the 7.62x51mm NATO.