We have Iran, Syria, and North Korea in the next few years. Do you we have time to make a major logistic change in time to fight these conflicts. To suggests that the 5.56mm is inadequate is rdiciulous. The Israelis use it, in fact, the weapon that OBL carried, or carries, fires a 5.45mm round.
The 6.8 SPC and, even better, the 6.5 Grendel use the same mags, lowers, action, in fact, everything but the barrel assembly. The 6.5 is way better than the 5/56, and so good, that in excess of 300 meters, it's better than the 7.62!!!
The .223 is 5.56mm x 45 mm the soviet version is 5.45x 39 and although it is more compact it has a fatter bottlenecked cartridge and has all the velocity of the .223 The actual bullet is much longer than the .223 and is a tumblin mofo when it strikes. I have AKS chambered for both and prefer the soviet round