At the moment velocity only affects bullet speed. That's all well and dandy, but generally speaking calibres are designed to use certain barrel lengths to gain the right velocity for proper fragmentation and penetration. 5.56 is the most obvious example. It's intended for use in 20' barrels and once you start chopping the length it loses performance until other calibres become more effective for the purpose of CQB, eg. .300 Blackout. There's a similar situation with certain Russian guns. For example the OTs-14 Groza originally had a 5.45×39 prototype variant that was quickly discarded when it turned out 7.62×39 and 9×39 just had better ballistic properties for the job of an SBR.
Anyway, I think this would be cool and encourage more use of niche calibres at shorter distances and maybe start seeing the intermediate cartridges more relegated to medium range. Also means that there'd be more of a functional difference between pistols and the 9mm SMGs/carbines and semi-auto rifles and bolt-actions. Of course, this would also bring the problem of the long-barreled no-recoil builds being yet even more effective, but I think that's moreso a consequence of how recoil is currently set up to function (but that's a whole can of worms for another repetitive discussion).
