I think that the assault rifle would be a dominant weapon on the WWII battlefield. There would have been a hard time with getting buy-in for the smaller caliber, but shot is shot, and you leave the battlefield. Higher rates of fire mean more chances to hit, so you either lighten the ammunition or build bigger soldiers. Having hefted a belt with several hundred 7.62 X 39 rounds I cannot wish to carry the same amount of .308 or 30-06 let alone 7.62 X 54.

I gotta stay with the .45 for close combat though. Nothing stops them in their tracks like a big slow hollow pointed boulder.

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PEOPLE SLEEP PEACEABLY IN THEIR BEDS AT NIGHT ONLY BECAUSE ROUGH MEN STAND READY TO DO VIOLENCE ON THEIR BEHALF. GEORGE ORWELL