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.
... from a short-barreled carbine. They had no problem with the 5.56 when the barrel was at least 14.5", witness the fact that the HK 416 remains the gold standard with the snake eaters. But those guys love their 10-in barrels, and that was the 6.8 SPC's strong suit.