It's highly unlikely there will be any change by the U.S. military away from the 5.56mm as its official caliber, at least for the next several decades. There's far too much investment already tied up into 5.56mm ammunition and weapons to abandon it. In banking terms, it's too big to fail. The 6.8mm SPC is clearly superior to the 5.56 in the AR platform, but that is essentially meaningless to the military (except for possibly SO use). The 5.56 does the job they want done for an infantry rifle, and that's that.