The .303 Magnum is an obsolete medium bore rifle cartridge.
The .303 Magnum was a bottlenecked centerfire rifle cartridge that was produced in both semi-rimmed and rimless versions. The cartridge fired a projectile of at . The cartridge's case capacity was the same as the .30-06 Springfield, although performance was considered to favour the .303 Magnum.
The .303 Magnum was developed by W. J. Jeffery & Co. by necking up the experimental .276 Enfield to , it was introduced in 1919 for target shooting and was used for some time by the British Match Rifle Committee. The cartridge had a brief life, only appearing in the Kynoch catalogue until 1930 and it appears to have become obsolete by 1932.