In jungle fighting against 90lb viet cong the 5.56/.223 is serviceable. Against 200lb sand people hiding in solid buildings and in stout vehicles, we must go back to 7.62/.308 or greater power. M14/AR10/FNFAL. These should be the minimum.
M4 size and caliber weapons in close quarter house to house fighting in the hands of a select few, could serve the purpose of a Thompson in the hands of a squad leader of WW 2 boots on the ground.
In an urban environment under 300m, I would take my AK-47/AKM (7.62x39mm) over the AR platform. Further than that, a bolt action 7.62x51mm or 7.62x54R would be fine.