The 6.8mm SPC would be the logical next step in the AR action for military use. Basically a shortened and necked down version of the .30 Remington. I was testing the AR-15 platform in 6.8mm before most people even knew about it and was very impressed. I still have a box of the first lot of 6.8mm ammo that Remington made. Some feel the 6.5 Grendel is in the same class, if not superior.