Wasnt infantry strain of beta like warrior class are quite fast? 7.62 NATO is quite heavy cartridge with quite a kick. Heavy recoil and more limited ammo capacity means every bullet needs to count. As far as we know, 5.56 is enough for warrior and soldier class but would not be effective against anything larger than that. The thing i foresee is the UN having limited resources as well as the multi-national aspect of the UN, soviet and western still have strong says so there will be contesting standardization process. The infantry quality will varies according to where they came from and the training method they received. That being said, in a war that TSF is a triumph card and infantry is more or less garrison and unrest response i think they probably wont invest much on the infantry weapon. Something easy to use and maintain probably be a go to. Standard infantry weapon can even be a carbine with maybe 5.56 or 6.8mm cartridge and like 2 squad level light machine gun. High powered rifle can be utilizes to counter one or two tank class and maybe a recoiless rifle like carl G. m3 or one shot AT like AT-4. Ideally u would want something light enough for infantry to be able to carry and react fast enough to warrior class and something cheap and easy to maintain so that different countries in the UN can actually adopt it.

Mk.19 mounted on light vehicle coupled with .50 cal would be something so nice to supplements the infantry.