In 1969 the U.K. started an examination to find the ultimate caliber for future use to replace the 7.62x51mm NATO round. The Americans were already switched to a new (smaller) caliber, the .223 Remington (5.56x45mm). The new round should have the same penetration characteristics as the 7.62x51mm, but with much less recoil of the firearm to be able to fire controlled automatic fire.
The 6.25x43mm was based upon the 7mm MK 1 round, necked down to 6.25mm. This case was only used as a test vehicle, the end stage of the cartridge should be longer and smaller.
Several different bullets were developed for the 6.25x43mm with different weights, all marked with blobs of paint on the head of the case to distinguish the different versions from each other.
6.25x43mm with 7mm Mk 1 and .223" for comparison. 6.25mm headstamps with red and green blob on the base.
In 1971 the tests were ceased in favor of the development of a new cartridge in an even smaller caliber, the 5mm (4.85mm).