Based on Overloads post the only thing REALLY differant on this is the barrel its self. With the tons of AR parts available it would seem to solve one problem, and reloading solves the other.
That's what I came away with as well. Which is why I'm so intrigued.
There's a tad more to it than that. Yeah, the barrel is heavier and different twist, 1-10.5 to 1-12 for the 6.5, don't recall the ones for the 6.8. And, if I remember correctly, the bolt group is beefier to handle "more power!". Mags need to be different too, but as I don't have a 6.x yet, not sure exactly how. I seem to have seen one statement that only the follower needs to be different, although a 30-round AR mag will hold less of the 6.x.
As for the rest, classic M-16/M-4 furniture and other little goodies - rails, sights, etc.