5.56 is not designed to kill. It is designed to be light, chamber in light, compact weapons, produce minimal recoil during automatic fire and to incapacitate targets, which it does relatively well when compared to submachinegun rounds. In short, it is a cartridge designed to exceeed the performance of pistol rounds while meeting the bare minimum performance standard for a battle rifle cartridge. A compromise within a compromise, if you will. It is not a serious cartridge for a serious battle rifle.
But in CW2 it is the weapon of the opposition and there will never be a shortage of ammo for it. So it has some value.