For 556, lighter probably carry 100 more rounds for same weight in 762, less recoil, faster follow up shots, easier to train green troops on, very accurate, in combat shoot a man until he quits moving.
For 762 or 308 better long range or any range cartridge but heavier more recoil, but more lethal.
For AK 47 or 74 great rounds, I only have the 47, and rifles, love my Vepr and milled receiver AKs.
In Nam we were all jealous of the LT who if I remember correctly was the only one who carried a CAR, original M4 back in those days, shorter and lighter.
Wish they would develop a bomb that temporarily blinded soldiers, maybe also causing hearing loss, maybe a giant flash/bang, if the enemy did not kill each other while being unable to see or hear, you could finish them off with a 22 long rifle or take prisoners.