it isnt that the 5.56 has less range 9(it has greater range in an sa80 when compared to the ak47).
it comes down to kinetic energy. the Russian round hits harder, ours does not.
however, our rifles carry more rounds, due to the smaller size. I have read recently that there may be a move over to 6.5mm boat tailed rounds for NATO. If this happens it will be a win