From The Manufacturer/Wikipedia/People Smarter Than Us:
LWRCI did some heavy lifting by redesigning the rifle to accommodate a cartridge offering more ballistic performance than the traditional 5.56 chambering. The round the company chose for its new rifle was 6.8 SPC. Optimizing the AR for a cartridge that falls between the 5.56 and 7.62 NATO is no small task. It called for taking the relatively small and light AR-15 platform and stuffing the larger 6.8 SPC round into it. And that’s just what LWRCI did.