In that case that sounds like 7.62 and ...... something.mr.fred wrote:You could almost certainly develop a single cartridge for infantry sections, but you may find that you would still want a heavier round for vehicle and tripod mounted weapons.
The 6.5mm and 7mm would seem like the obvious contenders but compared to 5.56, increased recoil is going to be a consideration again as will the modified training required to mitigate it.
Balistically the 7mm is king of the hill but very little penetrates like a 6.5mm especially if driven at high velocity due to its high sectional density. Could the dangers of over penetration in urban areas be a limiting factor?
It would be hard to develop a round that was optimised to penetrate body armour at reasonable distances and not increase the over penetration risk in urban areas.