Wrong community, mate. This is the Arsenal Football Club Community.

To partially answer your question though: 5.56 = mid-range fighting with long barrels; 9mm (or equivalents) = short range fighting for short barrels and immediate manstopping power. Everything also comes down to ballistics and 5.56 bullets need a certain length to not just tumble in the air after exiting the barrel. I know a thing or two but ballistic lessons where long long ago