As a combat round, I am of the opinion that if the military was looking for a good, well designed, intermediate round, they should have stolen the 7.62 X 39 cartridge and claimed it for their own. As much as I love the 308, it kicks too much for the little girls.
I'm no military historian, but -- to me -- the biggest advantage of the 5.56 cartridge is its weight. Ammo is freakin' heavy. The biggest problem with the 7.62x51 isn't so much that it "kicks too much for the little girls", but that the rifles are bigger and heavier and the ammo weighs double what 5.56 weighs.
I do have a bias: I earned my Distinguished Rifleman Badge with a M1a. Another issue, I never had to hump a weapon in combat and never shot a round in combat. Probably had hundreds of thousands of rounds go over my head, but I was nicely ensconced behind dirt berms pulling targets.
A shooting bud of mine, who is a Vietnam Combat Veteran took me to task as I was whining about the replacement of the M14 with the M16. He had carried both into combat and said you could carry 200 rounds of 7.62 or 400 rounds or 5.56 as a combat load and the advantage was, of course, to 400 rounds. I asked him if he had ever fired 400 rounds in a day, and he had!. So, there are some advantages to the Matty Mattel gun, but I will deign ever saying it.
Bud’s who have come back from combat have given mixed messages about the performance of the 5.56. One bud said no insurgent walked away after he shot them. Another, a Battalion Scout Sniper, said insurgents hit with the 5.56 round over 100 yards away, “just would not stay down”. He was however, very happy with the effects of the 7.62. Much better for a bang, flop, effect.
If you notice cartridge developments in the AR15, to improve lethality, have been pushing rounds back up to a 30-30 power level, which is the power level of the 7.62 X 39.
Given that I really love the 308, all of these subcaliber rounds are for ......