In Aftganistand it appears we need to return to the older infantry tactic of hitting what you shoot at. Which makes my old M14 a very good weapon to use. Does not put out the fire power the M16 does but it is very accurate at 500-600 yards maybe even 900 yards.
I take your point, and its a good one. The vast majority of engagements still occur at ranges of about 150-300 meters, so current 5.56mm rifles are more than adequate for that role. Designated marksmen and snipers need a different weapon, the 7.62 accurized and with match grade ammunition fills that role.
I believe that most infantrymen serving in Afghanistan would say that weight is their biggest problem. They have even more kit to carry than we did in Vietnam.