I have been looking at upgrades in caliber for the AR. There are obvious choices in 6.5 Grendel, 6.8SPC, 6mmAR, 458SOCOM, and 450 Bushmaster. Are there any upgrades that use the base 5.56 case or is it a moot point? Reason I am looking is I have a bunch of 5.56 P-Mags and any 6mm based upgrade...