When I was in the infantry, we spent more time training to support the Bradley Fighting Vehicle than we did developing light infantry skills. The Army made all 11M soldiers (mechanized infantry) into 11B (light infantry) soldiers but they still train soldiers to fight with, from, and for the BFV. That means months of training time spent at gunnery qualification ranges, field training exercises, and the NTC out in Fort Irwin. Rifle marksmanship got maybe 2 weeks out of the year.
Furthermore, our rifles were old and we would regularly have to shoot the old M193 ball ammo out of the M16A2 which means the bullets were deforming in the barrel(hardly a recipe for developing accuracy). Even if we were shooting M855 ball ammo (the green tipped AP ammo designed for the M16A2), it isn't capable of consistently shooting 1" groups at 100 yards (1 MOA). It is capable of shooting 4 inch groups at 100 yards (4 MOA).
So, if you can only shoot 4 inch groups at 100 yards, guess what that means? It means you're only able to shoot a 16 inch group at 400 yards. That means, you are going to miss your enemy target at 400 yards under all but the most perfectly optimal of conditions (conditions which never occur in combat). Bottom line, the ammo sucks and rifle marksmanship training takes a back seat to too many other training needs in MANY infantry units.
True, at the end of the range, the heavy bullet will always be more accurate and do more damage. Less wind drift!!