The .303 British Ammo, also known as the 303 British or 7.7×56mmR, is a rimmed rifle cartridge with a .303-inch (7.7 mm) bore diameter. It was initially developed as a black powder round and introduced in December 1888 for the Lee–Metford rifle. In 1891, it transitioned to using smokeless powder, which had been the original plan. However, the decision on which smokeless powder to adopt was delayed.

The .303 British cartridge served as the standard ammunition for rifles and machine guns in the British and Commonwealth military from 1889 until the 1950s when it was replaced by the 7.62×51mm NATO cartridge.


Over its service life of more than 70 years, the .303-inch cartridge underwent various developments and had around 26 different variations, designated by ten marks. Compared to other service rounds of the early 20th century, the .303 British had relatively low bolt thrust.


The propellant used in the .303 British service cartridge was black powder. It was specifically adopted for the Lee–Metford rifle, which had rifling designed to minimize fouling caused by this type of propellant. The .303 cartridge replaced the Martini-Henry rifle in 1888, and some Martini-Henry rifles were rebarreled to use the new .303 ammunition, resulting in the “Martini–Metford” configuration.