And, the US military is currently adopting the MCX Fury as their new standard issue rifle.
That thing fires the .277 fury cartridge that is way better at armor penetration than 5.56 ever could be.
And the reason .308 isn’t used as much for most military issued rifles is more to do with recoil and capacity than anything to do with the ballistics.
A .308 would be far better for killing literally anything than 5.56, but is mostly for use in semi-auto marksman rifles and some snipers. Where accuracy of first shot matters more than volume of fire.