I have come to the conclusion that our military's search for "one gun that does it all, equally well" is like the 17th&18th century search for the Northwest Passage. Only without the shiploads of people freezing to death or starving locked in the ice.
I keep coming back to the model that we wound up finishing WWII with. Not talking about the overall mechanical excellence of the firearms used (thought there is that), but the mixture of arms supporting each other in infantry combat.
A good solid pistol, a heavy caliber "thumper" SMG or shot gun, a longer ranged, light carbine, handy and quick, a full size rifle with even greater range and power, and "light" automatic weapons support using that same round.
Arguably not the best possible combination, but one that did prove successful, along with all the other support we could provide such as heavier fire support, and logistics support that ranged up to levels our enemies considered lavish luxury.
I bow my head in respect to all those who were in places and at times when our supply side failed them, and they got their job done, anyway.
The general mix of small arms we used in WWII worked pretty well in all environments, though some arms effectiveness would vary with terrain and climate conditions.
Somehow we went from that to "everybody gets the Mighty Mattel, except for the pig and the blooper".
Today, we've come some a long way from that, again coming back to the concept that "maybe" having a bit broader mix of weapons isn't such a bad idea, after all.