From my comment on Military Arms Channel's video, Goody bye M16! Military Adopts the M5 rifle and 6.8x51. Are they crazy? - YouTube
Hold off on going 6.8x51 till it's confirmed by testing and deployment. It indeed will be several years. There were NG units still using Garands as late as the 1970s (Kent State). Think of all the folks who departed from common North American and NATO-standard calibers by investing in 6.8 SPC. *suppresses laugh* I do wonder how many of the naysayers are just mad they got caught holding that bag.
But presuming it happens, learn from past overreaches: Keep things simple. Don't go for the 2A hobbyists' approach of maxing everything out. Don't go for the CA folks' approach of trying to skirt every AW law, and thus encourage more such laws. (Remember, most Americans support gun rights, but disagree with the purists on some points. You can say that's contradictory, but the political reality--and in some cases judicial precedent and common law going back centuries, even pre 2A--are on their side). A simple, rather featureless fighting rifle in the (new) standard military caliber (and NATO will come along, as they did with 5.56) will serve the function of the civilian 2A Militia and their most likely employment. But if one must go with it now... The SIG CROSS bolt-action is available in the caliber. Suitable for set-position and citadel (think Mount Carmel) defense.
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