The Navy and Marines tried a 6MM in the 1890's but it lacked modern boat tailed bullets, They liked it OK, but the Lee rifle was to delicate and complicated for service.
The Soviets went with 2 calibers 7.62X 39 for individual weapons and Squad automatic and 7.62X54 for Sniper rifles and heavier machine guns, it works but complicates the supply chain, (even more so when the pistol cartridge was also a 7.62 X25 Tok ).
So twice in a row I have to agree with TAB.
The .308 is technically a known commodity, loads, bullet wts, barrel twist rates, have all been worked out years ago.
It deprives the individual soldier of the ability to spray and pray, but conventional troops do not need full auto fire. they need to know how to shoot.
Full auto is appropriate for specific military purposes and is supplied by dedicated machine guns that are designed to handle the stress's, heat, and recoil of sustained fire, that can be chambered in the same caliber simplifying supply.