You know I was mostly with you until the 6.8 rant, that is the same bullet as the 270 Winchester probably the most numerous chambered hunting rifle after the 30-06 and yes that is taken into account the gobs and gobs of 7mm magnums and Mausers out there. Finally the 6.5 just hasn't really caught on this side of the Atlantic and I don't see the Army every adopting a straight walled case, the is going to be at least a slight taper so the Grendel is out, IMO.
Then we are still together. I'm not that familiar with MM so I wasn't aware that it was the same caliber as the .270 which I have taken a number of game animals with and it is an excellent bullet and the round its self one of my favorites.
When considering, weight, recoil, range,accuracy and penetration I considered the 260 with it's 120 or 125 gr bullet Rem.=6.5MM=.264in. to be about ideal though you are right it has never been very popular over here. However the .270cal.,=6.8MM=.277in.with its 130gr. would be just as good if not better as the B.C. would be about the same.
They should be able to design a case as done with the 7mmTCU or 7mm BR and still get 2500 fps.and allow the carrying of more rounds.
You are right I can't see them using a straight walled case when chambering and extraction seem to be better with tapered or bottle-necked cases.