The reason the 5.56 was adopted was because the 5.56 is light weight, compact, cheap, and has very little recoil.
This means that a soldier can carry a lot more ammunition for less weight (more than twice as much as with 7.62 nato), and less money; and they can fire their weapon a lot faster, and more controlably (especially in full auto). Also, the weapon itself can be shorter, lighter, and cheaper; because it doesnt have to resist the forces of a larger, more powerful round.
Yes, we adopted the 5.56 with a flawed wounding model, but in fact McNamara and the DOD didn't care... at all. What they were concerned about was that the 5.56 was light, and cheap. A soldier could carry 2.55 times as much 5.56 as 7.62 nato, but they cost the same.
Combined with the incorrect wounding model thought to apply to the round at the time, and the defense department (though not the military itself, especially the Army Ordnance Board), thought it was a win-win situation.
Well, it turns out they were wrong; and they knew they were wrong pretty quickly.
And now that our last 20+ years of conflict has been at , predominantly, street distance...the 5.56 is even more flawed.
A slower .30 isn't the absolutely answer but its an attempt.
My thought is a 7mm bullpup with fiber optic 2x and a 650-700 rpm ROF.
