The British 0.303 Cartridge (metric calibre 7.7 x 56R) is a rifle calibre bullet with a rim diameter of 0.540in and a body diameter of 0.460in.
There are three main varieties -
Mk2 ball weighing 13.9 grams with a muzzle velocity of 610 m/sec.
Mk7 ball weighing 11.27 grams with a muzzle velocity of 744 m/sec.
Mk 8 'boat tail' ball for Vickers Machine Gun, weighing 11.27 grams with a muzzle velocity of 777 m/sec. It was also used in the Bren Gun.