I think the SEALS use the updated M-14 plus other units are using it for longer ranges plus suppression. So you got the long range plus extra ammo.
Suppoedly, during Nam, the MVA and VC avoided the Aussie troops (yes Aussies had troops in Nam) becuase they carried FAL FN’s with 20 rounds of .308. A bigger rifle to hump in the jungle but more firepower than the M-16 and probably better relibility.
McNamara should burn in hell.
In the sandbox the M14 type of rifles fill in the requirement of the mid range one shot kills that the .556 would be at its limits. The 60’s era thought was that the enemy would tie up resources for wounded combatants, thus the M16 and its smaller bullet was evolved into use. However modern day enemy have no such regards about saving their fellow soldiers so wounding one only gives them some extra time to shoot some more or be a suicide bomber.
The .308 with a nominal 150-168 grain bullet is many times more powerful than the .556 or even the 7.63x39.