One of the lessons learned by the British military in the aftermath of the Boer War was that modern Mauser rifles were superior to their Lee-action rifles and carbines. In response, British ordnance began experimenting with a Mauser-pattern rifle, ultimately finalized as the Pattern 1913. This rifle would also leave behind the obsolescent .303 rimmed cartridge, in favor of a new rimless .276 Enfield round.
The Pattern 13 rifle itself was excellent - it balanced and handled well, it had very good sights, and a smooth and fast bolt throw. However, the .276 Enfield cartridge was really more potent than it needed to be, and caused problems. The cartridge threw a 165 grain bullet at just under 2800fps, pretty close to the ballistics of today's 7mm Remington Magnum. Loaded with Cordite propellent, this led to excessive barrel wear and unpleasant recoil, along with some parts breakage. However, as final testing was being done in the first half of 1914, the Great War broke out.
At this point, plans for using a new cartridge were abandoned. The rifle itself was redesigned in the .303 cartridge, to be manufactured in large numbers by American firms under contract. It would also be refitted for the .30-06 cartridge and used in large numbers by the American armed forces as the M1917 Enfield rifle. According to General Julian Hatcher (who ought to know), it was the best rifle of the First World War.