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At the end of WW2 the British were one of the few major powers to not have a self-loading rifle in service. They were still using the .303 British No. 4 Lee Enfield dating from the late 19th century. Combat experience during both world wars questioned the need for infantry weapons that were accurate out to 2,000 yards and the evidence showed that most situations were at short range, where the need was for a weapon with selective fire with an effective range longer that a submachine gun, but shorter than a conventional bolt action rifle. The British Ministry of Supply set up “Small Arms Ideal Caliber” panel at the end of 1945 to determine the optimum cartridge for a light weight self-loading rifle.

A great deal of experimentation and testing took place, primarily of calibres between .250 and .270 (approximately 6.35 to 6.8mm), undertaken at the Armament Design Establishment, Enfield under Dr Richard Beeching, the Deputy Chief Engineer. The panel reported in March 1947, recommending the further development of two different designs. The first, appearing in November 1947, was in .270 calibre (6.8x46mm) with a steel-cored 100gn bullet travelling at between 2,750 and 2,800fps (approximately 840 to 850m/s). This round retained 81 ft lbs of energy (109j) at 2,000 yards (1,830m) with 60 ft lbs of energy (80j) reportedly being necessary to injure an unprotected human being. The second was in .276 calibre (7x43mm), later re-designated as .280 to avoid any confusion with earlier ammunition, such as the American .276 Pedersen and .276 Enfield P13, both of which had been considered as potential new rifle calibres for their respective countries, in 1913 and 1932 respectively.