6.5 Grendel IF you are going to keep the Ar- M4 platform- good out to 600-800yds on man sized target and plenty of penetration to get into and through stuff-
or M14 SOCOM - with a folding stock and 4/6 X scope good enough for the grunts --done!
BUT first we have to teach "kids" to SHOOT well before they go to BOOT- or worse over seas
'head shoots' mitigate the 30 shots to the chest thing -
ONE in the brain box- then two in the chest- Over kill?NOPE - dead is dead-
I mean, I don't really mean this to be taken offensively I suppose, but you sound like someone who plays a bit too much video games and watches a few too many Army movies, and not enough real life training and trigger time.
Also, its against the Hague convention to do headshots, as they are considered to fall under "indiscriminate fire" since the miss probability is high, and a high probability miss could result in a civilian casualty, directly against the rules of war.
These are my ammo reserves that started in Jan and will last me until about summer... I have years of training under my belt, and I still would never rely solely on hoping to get head shots. Especially when you take into it elevated heart rates, less than ideal conditions, being shot at (you're only going to shoot someone who is shooting or threatening to shoot at you, right?), distance, wind, luck...
Also, the magnification scope is not necessary in the Army's view.. most engagements are within 300 yards, and you can open sight that range and a scope is an added material/maintenance cost.
Also, you'd NEVER adopt a 6.5 Grendel for military or LE use. They tend to shoot out barrels after ~5-10k rounds of ammo MAX (assuming good care of the barrel).
Usable service life for a military service military service M16 is ~10-20k, for the HK416 is 50k. They wouldn't take steps back in reliability, even if it scored points in lethality. That math isn't how Congress looks at things.
They are looking like they'll eventually adopt 6.8SPC POSSIBLY, though the 300blk is so popular these days and the fact that it requires so little retooling for mags, bolts, gas system, etc, it is the most likely contender in my book.
I doubt they'll make much if any changes to big Army anytime soon in terms of cartridge though, lack of funding an all.