According to article in "Special Weapons" magazine, the main reason for the Military searching for a new caliber is lack of performance of the .223 in the M-16's and the light machine guns of same caliber. It was a combined effort of SOCOM and Remington to develope the 6.8SPC to possibly replace the .223 in military small arms. There are many other 6mm calibers also being looked at, with no replacement chosen as of yet.
A major drawback to the 6.8 SPC is that they CANNOT be used in current M-16 magazines and they CANNOT be used with current M249 links, due to the increased case diameter. It will also require extensive weapon modifications...ie new bolts, magazines, barrels and links. There are others that can be used with only minor mods to the magazines and will fit in current links...300 Whisper, 6.86 ARC, 6mm-223 are also being looked at.