303 British Ammo (designated as the 303 British by the C.I.P.and SAAMI) or 7.7×56mmR, is a .303-inch (7.7 mm) calibre rimmed rifle cartridge. The .303 inch bore diameter is measured between rifling lands as is the common practice in Europe which follows the traditional black powder convention.
It was first manufactured in Britain as a stop-gap black powder round put into service in December 1888 for the Lee–Metford rifle. From 1891 the cartridge used smokeless powder which had been the intention from the outset, but the decision on which smokeless powder to adopt had been delayed.It was the standard British and Commonwealth military cartridge for rifles and machine guns from 1889 until the 1950s when it was replaced by the 7.62×51mm NATO
History and development
During a service life of over 70 years with the British Commonwealth armed forces the .303-inch (7.7 mm) cartridge in its ball pattern progressed through ten marks which eventually extended to a total of about 26 variations.The bolt thrust of the .303 British Ammo is relatively low compared to many other service rounds used in the early 20th century
.303 British Ammo service cartridge employed black powder as a propellant, and was adopted for the Lee–Metford rifle, which had rifling designed to lessen fouling from this propellant, which replaced the Martini-Henry rifle in 1888. Some Martini-Henrys were rebarrelled to use the new .303 as the “Martini–Metford”