.303 British, or 7.7x56mmR, is a .311 inch calibre rifle

Rifle

A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...

 and machine gun

Machine gun

A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute....

 cartridge first developed in Britain

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 as a blackpowder round put into service in December 1888 for the Lee-Metford

Lee-Metford

The Lee-Metford rifle was a bolt action British army service rifle, combining James Paris Lee's rear-locking bolt system and ten-round magazine with a seven groove rifled barrel designed by William Ellis Metford...

 rifle, later adapted to use smokeless powder

Smokeless powder

Smokeless powder is the name given to a number of propellants used in firearms and artillery which produce negligible smoke when fired, unlike the older gunpowder which they replaced...

s. It was the standard British and Commonwealth

Commonwealth of Nations

The Commonwealth of Nations, normally referred to as the Commonwealth and formerly known as the British Commonwealth, is an intergovernmental organisation of fifty-four independent member states...

 military cartridge from 1889 until the 1950s when it was replaced by the 7.62x51mm NATO.
The measurement 0.303 inches (7.7 mm) is the nominal

Real versus nominal value

In economics, nominal value refers to a value expressed in money terms in a given year or series of years. By contrast, real value adjusts nominal value to remove effects of price changes over time...

 size of the bore measured between the lands which follows the older blackpowder nomenclature.