The question of this thread is short-to-medium range sniping.
The 6.8 SPC is a definite no-go. Look at the latest Guns & Ammo issue with the tested 6.8 SPC...100 yard groups of 4 to 5 inches. Medium range would be out to 500-600 meters and you would be shooting 20" to 30" groups with this cartridge. Search posts by those who own the 6.8 SPC and you see 95% of them going well over 1 MOA in accuracy. The 5.56 is much more accurate.
For energy delivered and sniping the 6.5 Grendel is the way to go. Again, search posts by users and the majority of 6.5 Grendel shooters are getting 1/2 MOA at anywhere from 100 to 800 meters. There is also an article on the 6.5 Grendel in the latest Shooting Times (Mar 07) and groups are 1/2" at 100 yards.