Well, part of the issue is that as compared to missiles bombs, and artillery, all rifles are marginal man-stoppers.
You have a very tiny bit of lead and copper that has to intersect a moving human who's not cooperating with your efforts to shoot him. The world is big, people are small, and bullets even smaller.
Often "hits that fail to stop" aren't even hits at all.
And sometimes people who do get hit don't know they're supposed to politely fall over and die either. When it happens to our guys, it's heroic. When it's theirs, it's an "ammo failure".
And of course, we could give every trigger puller a cut down Barrett .50 BMG, but it would be impractical. We could even give everyone a 7.62 NATO rifle again. But instead we'd be bemoaning the fact of how often we ALSO get into CQB and MOUT and it's unwieldy. Or how a unit that got cut off momentarily couldn't hump enough ammo.
It's always a compromise. And there is indeed merit in the ideas behind the 5.56. Firepower, weight, control-ability, flat trajectory... But it comes at the expense of terminal performance. You up the terminal performance with larger rounds, and the others suffer.
So getting into the intermediate 6-7mm neighborhood, getting back some terminal performance, and keeping some firepower/weight/flat shooting might the way to go. Of course the logistical concerns of doing so are not small.
