Testing, testing...

[stanc]

The barrier penetration doesn't have to match M80 at all ranges, just be closer to 7.62mm than it is to 5.56mm.

Similarly, I don't think that the muzzle velocity, or energy retention at long range, need to match M80 exactly as long as they are in the same ballpark, so that there will be little practical difference as far as the troops are concerned.

It seems to me that barrier defeat and wound trauma must be equal to 7.62mm, or both the troops and the decision makers will object to replacing 7.62x51 with a less effective round. If we were still talking about replacing 5.56x45 only, that would be a different matter.

Also, if muzzle velocity (and to a lesser extent, downrange velocities) don't match M80, it won't pass Joe's "that looks about right" test. It isn't possible to match the retained energy with a lighter bullet launched at the same speed, so I'm not concerned with that. If terminal performance is as good as 7.62mm, energy figures shouldn't be of much consequence.