5.56 is so weak that US Army is switching to a 6.8x51 higher pressure round beginning in a few months. Sig-Sauer is building the new XM7 rifles.
The Spec Op’s guys wanted a round with more punch than the 5.56mm, hence 6.8mm Remington SPC, special purpose cartridge. Out of a Barrett REC7 it makes for a nice platform. Still prefer the 7.62 round though.