One question; as XCOM is a paramilitary organisation, would you allow non-military recruits? So security operatives, police, African mercs etc etc.
And as an ex-Royalie, 7.62 chambered rifles would have made the world of difference in Afghan. Using the ANA's AK-47s, we often found that they were more effective than our own 5.56 SA80s when fired semi-automatic.
In this context, you wouldn't want to bring a 5.56 rifle to fight a Muton. Just no. But that doesn't mean that 5.56 doesn't have it's uses; it would be more handy when fighting in rural terrain or in open spaces. But in urban combat or when assaulting a UFO, a 7.62 is the way to go.