I'm thinking about building an AR15 rifle sometime in the future. I know that 223 Rem/5.56 NATO is the caliber that dominates the AR platform. But how likely is it that one of the other calibers (224 Valkyrie, 6.5 Grendel, 6.8 SPC, 300 Blackout, etc) will become the new normal for the AR15 platform? I haven't followed it too closely, but I did hear the military was thinking about replacing 5.56.