Like most armed forces in the immediate post-World War II era, the British Army began experimenting with lighter rounds after meeting the German StG 44 in combat.

Efforts to replace the .303 with a more modern round predated even World War I, but a series of events kept it in service in spite of its rimmed design causing a number of alleged problems[citation needed].

This led British small arms designers to begin the development of their own intermediate round, but increased the range requirement so as to be effective out to 400 yards.

[3] They suggested that the ideal cartridge would be .276 to .280 inch (6.8 to 7 mm) in diameter, weigh between 130 and 140 grains (8.4 to 9 g) and have a muzzle velocity of approximately 2,400 fps (730 m/s).

The .280 cartridge was developed under the direction of Brigadier Aubrey Dixon, and approved for testing in mid-1947, a similar design, the .270, was released in October of the same year.

In 1949 the rim diameter was increased to .473 inch (12 mm), the same as the United States' standard Caliber .30 ammunition, and redesignated .280/30, the bullet was changed to a heavier 139 gr (9.0 g), with a mild steel core still at 2270 fps.

In early 1950, the .280 cartridge was tested by the Development and Proof Services at Aberdeen Proving Grounds, Maryland, and the results were disappointing.

By September 1951, the NATO Standing Group agreed on a set of military characteristics of required performance for new small arms ammunition.

[7] A change of government meant that the 7 mm, EM-2 and Taden gun projects were abandoned soon afterwards by Winston Churchill, who returned as the prime minister who desired commonality between the NATO countries.

Large caliber bullets were calculated to need more energy to penetrate various levels of body armor to inflict disabling wounds on soldiers.

While the 7.62×51mm NATO required 700 joules (520 ft⋅lb) of force on impact to penetrate helmets and heavy body armor, the 6.25 mm required only 580 joules (430 ft⋅lb) of impact force to deliver the same penetration effects out to 600 m. It remained effective for a longer distance and produced recoil closer to that of the 5.56×45mm NATO.