I had the pleasure of following a line of discussion about a replacement for the M4 a little while back on another forum, and it was most illuminating in that one of the participants had seen the XM8 on trials and was happy to describe it as no great improvement over the M4 once the external factors in terms of maintenance had been eliminated. In other words, treat both weapons with similar care or lack of, and the differences soon narrowed to very slender ones. Given you'd have to issue a whole new line of weapons, accessories, run training for all troops, certify armourers, lay in a line of replacement spares etc, the incoming weapon is going to either have to be a remarkable leap forward, or the outgoing weapons will have to be quite badly worn.