The M14 7.62mm Rifle was replaced in the sixties with the M16 5.56mm for a wide variety of reasons not the least of which is the additional amount of ammo a troop can carry into battle. Also the m16 seems to tie up enemy resources by delivering an incapacitating wound rather than certain death, or at least that’s how the theory goes.
So much for theory. Read “Blackhawk Down”. The attackers were hit with multiple .223 rounds from the poodle-shooter and kept coming. The guys shot with the .308 went down and stayed down.