First of all, there is the issue of compatability with the ammunition and weapons used by the bulk of our enemies around the world. We don't want the enemy capturing our ammo and even our weapons to use against us. 7.62x39mm Soviet is all over the place in third world countries, but I'm guessing that 5.56x45 NATO is a little harder to come by, unless it's off the bed of a U.S. supply truck. So if our enemy captures out weapons they are not worth hardly anything, same goes with ammo.

Secondly, the 7.62x39mm Soviet is already loaded with as light a bullet as is really practical in a .303cal cartridge. The standard load is a 123gr FMJ at about 2300fps. Short range performance is pretty good, but out past 150 meters it's falling like a rock and has little energy left. The 6.8mm SPC still has what it takes if ranges get large, but is a powerful and useful cartridge up close as well. Burst fire might be a bit tricky to control, but when you're 25 yards to 10 feet away from the enemy, it shouldn't be a terrible problem.

Mike