In my opinion the amunition the troops use has to do two things good.
To reliably kill humans and to have some barrier penetration capability so it can get to the human hiding behind it.
With a small caliber, because of the low weight, you can't do both this things great. You can do one of them great and the other one bad, or try to do them both so-so.
First the 5.56x45 was doing the killing good, but had really poor penetration.
After some modifications, the penetration was improved but because of that bullets are now more stable and don't fragment as reliable. Without early enough tumble and fragmentation you are just left with a small .22 hole in the bad guy. Unless something vital was hit, that small hole won't drop the bad guy and he can still fight.
A larger caliber, heavier modern bullet, has a lot more chances to do both much better. More mass means higher kinetic energy and momentum to help with both
1)better barrier penetration and
2)more and bigger fragments to damage tissue and kill the bad guy faster.
Sure most of us here in the west hemisphere would stop fighting right there and then if we got hit by a non fragmenting 223, but we find our troops fighting jihadists this days.
This guys are there to try to bypass the slow and painful "going to heaven system".
Think about it. This guys live in some bad, poor places, all they have is the Islam.
They got tired of the hard life they have to put up with every day so "martyrdom" looks like a quick way to change the crappy hard life (most of this poor guys have to put up with), for heaven.
This guys are not there sent by someone by force, they are there to kill our troops and die as martyrs in the process. Going home wounded is not what they are looking for.
So many of them don't stop fighting when the heaven is so close.
That's why now I think a heavier bullet would be a lot better against this type of enemy because modern bigger bullets, like 6.8x43mm, DO kill faster.
Just like the appearance of the kamikaze planes made the navy to arm a lot of their fighters with 20mm cannons instead of .50's. The fifties did kill planes, specially unprotected/unarmored Japanese planes, but the quad 20mm package killed them a lot faster.
We all saw plenty of WW2 footage with burning kamikaze planes crashing onto US ships. Is a bout the same with the enemy our troops face this days.
I also think that against the enemy we face today the care for the wounded enemy combatants is falling on our friendly forces more then we would like to admit.
There are plenty wounded enemy combatants treated out of our money.