5.56mm

Soon after the United States began re-equipping her forces with the .223 inch AR 15 rifle in the early 1960s Britain naturally took an interest in the new rifle. It was ironic that only a few years earlier the Americans had resolutely refused to consider the British 7mm round as the future NATO cartridge, saying nothing less than a .30 calibre round would suffice.

In 1965/66 Britain purchased 6,000 Colt AR 15 rifles for evaluation and use by special forces. The new lightweight rifle was of particular interest because Britain at that time was heavily involved with the campaign against Indonesia which was being fought in the heavy jungle of North Borneo. Later Britain purchased further M16A1 rifles in 5.56mm (as the calibre had now been renamed)

American ammunition was bought for use with these rifles but small scale British development of 5.56mm ammunition followed soon after.

Britain spent most of the 1970s developing first the 6.25mm round and then the 5mm/4.85mm round in preparation of the forthcoming

trials for a new NATO standard ammunition. Ever hopeful, but aware of the realities of the trials, Britain was careful to base the 4.85mm round on the 5.56mm cartridge so that the new infantry rifle could be manufactured in either calibre.

From 1977 to 1980 NATO held trials for the new common cartridge which unsurprisingly was won by an improved version of the 5.56mm round. Britain's hope of success with the experimental 4.85mm round was dashed and so the fall back position of modifying the SA80 rifle to 5.56mm was adopted and as the L85A1 it became Britain's new infantry rifle .

Development of the 5.56mm round in Britain has continued and is likely to do so for some time into the future.