M14 = 7.52 x 51 Nato aka .308 Winchester but still has more thump than 5.56.I think we still have a good answer in the army inventory already, the venerable M14 (30-.06 ) comes in a real powerful armor piercing and there are scads of them in armories. Just takes a little more muscle to carry around and a little better marksmen so that they don't need to carry so much ammo.
That's what I'm saying Coot! The .308 is fine and all, but I'm not so sure that a heavy loaded 45-70 wouldn't be a much worse proposition to be on the receiving end of......Just for the fun of it, give some thought to what it would feel like to be on the receiving end of a 450gr bullet poking along at 1600fps, mid-chest!
A person might not care if the armor stopped the boolit or not.
Think the biggest horse you have ever seen, planting both hind feet center chest multiplied by about ten.
Kind of like when I did a face plant while knee boarding. Like my daughter said, it hurt so bad if it wasn't for the life jacket I wouldn't even bother to come back up!
Crusty Deary Ol'Coot