Standard rifle for China QBZ-95 uses 5.8x42mm cartridge. The AK-74M replaced the AK-47 and uses a 5.45x39mm round.
The applications are different in combat and hunting. If you had multiple hostile deer wearing body armor trying to shoot back at you, 30 round magazines of high-velocity, low profile rounds might just be exactly what you need.
Well, no kidding. How much have you spent in combat? As far as the Chinese and Russians adopting itty bitty calibers - swell - I don't mind it all if our adversaries adopt bad ideas. But close range "pray and spray" isn't effective in most fights. It's those rare folks who aim their fire carefully that win battles.
The M-16 family is NOT robust: it's aluminum and plastic and the breech is deeply enclosed and inaccessible. It gas system fountains carbon into the bolt and breech and the whole thing bends with very little effort. It is and was an inexpensive weapon to produce and field and the army is loathe to look at anything else, no matter what the field experience shows. During the height of the Iraq War we had requests from commanders in direct combat for M-14s because of the ineffectiveness/imprecision of the 5.56 round at longer ranges/penetrating brick and earth walls/lethality but those requests were turned down because stateside clowns with zero combat experience were determined to keep going with M-16s only.
There are many excellent designs out there and much better calibers yet the army establishment is perfectly happy to keep the status quo - or offer idiotic alternatives like that XM-25 - a beast in no way suited for the realities of infantry combat.
It's time to put veterans with combat experience into weapon development programs over the civilian developers at Picatinny.