But I think if they could develop a round that is small and expands, it would be better than a "heavy" round with lots of recoil.
I would make a expanding small round with no recoil and with 100 or 200 round magazine capability.
But then there is the Geneva Convention on ammo to that prevents that.
The 5.56 in the M16 platform has practically no recoil.
The 5.7 round is probably what you're looking for, putting 50 rounds in a very compact CQB package. That's probably the best power/capacity tradeoff you're looking for.