The 7mm second optimum:
In 1952 after the NATO trails it became clear that the United States would never adopt the .280/30 Enfield round.The reason for this refusal was the lack of power compared with the .30-06 round (the service round of the U.S. army at that time, and used as a reference) and the American experimental T65 cartridge.
In an attempt to increase the performance of a 7mm round the engineers of the Belgium factory FN (Fabrique Nationale) (as member of the BBC committee) developed the 7mm 2nd optimum. The American FAT1E1 case with a length of 49mm (necked down to 7mm) was used together with the belgium 7mm Mauser S12 bullet.
The 7mm 2nd optimum was only produced in Belgium and the United Kingdom.
7mm second optimum, (.280/30 for comparison), headstamp FN 56
The 7mm 2nd optimum was also produced in different loadings, including a grenade cartridge. (see image)
7mm second optimum ball and grenade cartridge headstamp RG 53
After the adoption of the 7.62x51mm cartridge by NATO, FN sold their 7mm second optimum as 7mm Liviano to Venezuela, together with a FAL-rifle in this caliber.
The Amour Piercing round showed below is probably used in the Belgium attempt to sell this 7mm Liviano to Venezuela.The headstamp of this round is F N 56.
7x49mm with A.P. bullet