Love both cartridges. In the jungles ov Vietnam the .223 was mostly sufficient. In cities shooting through buildings and cars and for open plains the .308 would be my choice.
The beauty of the .223 is the amount of ammo you can carry. My dad during WW 2 had no problem with his .30 carbine ( a fairly mild round) in the swamps and jungles of the Solomons.
For long range and for power to penetrate I’d personally prefer the .308.
The AR was designed for the .308. The AR10. It is much lighter than the M-14. Has the same punch.
Both rounds can be used by the same squad if supply chains are properly run. I think using both is the way to go.
No, it wasn't: way too often the damn thing just knocked dust off them and they kept running. When the rifle worked.