6.8mm SPC all the way baby!
6.5 Grendel is superior in every way.
Well, according to the article “No caliber restriction has been placed on a new design. It will be at least a 500-meter weapon and have a higher incapacitation percentage”
That rules out 5.56 and the brass still thinks that 7.62 is too heavy ... so something in the 6 - 7 range is likely to be the choice.
I personally favor 7 x 51 (7-08 Remington) but the 6.8 SPC would do just fine.