I own ARs in 223, 6.8SPC and 308.
The AR-10 size is for the 308 and other shells bases on it I have made about 25 AR-10s in 243, 260 7-08 308 and 338 federal. All were accurate and functional, but all were on the heavy side. My own "10" is a Ruger SR762 with with a 10 round mag, scope and mount is still 11 pounds.
The AR15 "Deer Rifle" is best in either the 6.8SPC or the 6.5 Grendal. The 6.8 is my choice, but there is not much difference between the 2 out to about 350 yards. I have made about 20 6.5s and about 35 6.8s. I have loaded a lot of ammo for both and I find that with the same barrel lengths the difference is so slight as to be unnoteworthy.
The 6.5MM bullet fly a bit flatter the .277" bullets at longer ranges, buck wind a bit better. But in my experience I have seen kills with the 6.8 that were faster and more "eclectic" then those I have seen with the 6.5.
If you like 16" barrels I have found the 6.8 is a bit better then the 6.5G. If you have a gun with a 20 or longer barrel, the 6.5 out preforms the 6.8 by a small margin across the board.
I hear a lot of tripe about the 800-1000 yard hits from the 6.5G and the stories are largely true, but these things are answers to the wrong question.
Yes the 6.5 is easier to hit with at 500 yards and farther. But neither shell is made for killing game at those ranges. Remember, both are intermediate size shells. Neither will come close to a light bolt action in 243 or 25-06 at 500 and out. In the cases of the 22" and 24" 6.5 Grendels I have made, they are heavier and longer than the nice bolt action Mausers I have made in 25-06 270, 280, 30-06. This takes nothing away from the 6.5 G, but if we are talking about hunting rifles I believe we loose sight of the mission statement when we make them long and heavy for deer and antelope. Yes they are excellent tools for the job, but not quite as good as what we have had available for 100 years in lighter guns with flatter shooting and more powerful cartridges.
I personally have killed a lot of deer and antelope with my 6.8 and a few with 6.5G, and I have seen a lot more deer and antelope killed with both. Both were made to out-kill the 223 at every range and both do that very well. But in my opinion the 6.8 is a 400 yard shell, and 500 in the hands of superb marksman. With the 6.5G I'd add 100 yards to these capabilities for deer.
At ranges from "pistol distance" to about 300 yards I have seen a slight edge in legality of the 6.8 over the 6.5, but not enough to argue over.
Most Grendal shot deer and antelope have run 2-3 seconds longer than the ones I have killed with the 6.8. Most 6.8 shot deer and antelope have been bang-flops. The 6.5s (so far) have not been as good at breaking bone either.
I see more good 6.5MM bullets being made today than I had to use a few years back, so the 6.5G is closing that gap if loaded right. Nearly all 270 bullets were made for hunting. Many of the 6.5MM bullets were made for paper. So some of the comparisons are unfair because the 6.8 had a far better bullet in it for killing things than the 6.5s did. The best one I have used (and seen used) so far in the 6,5MM is the Nosler 125 grain Partition. For deer I doubt you can beat that bullet.
The longest shot I have made with either was with my 6.8 at just over 400 yards. The longest shot I have seen made was also with the 6.8 on a deer at about 440 yards (437 according to the laser) Both these deer dropped at the shot. The longest shot I have seen made with a 6.5 Grendal so far was one hit at about 285 yards. It didn't drop, but only ran about 20 yards.
As I said, I choose the 6.8 over the 6.5 for personal reasons, namely that if I want "long range capability" as a primary concern, I already own several 270s, a 25-06, two 308s, two 30-06s, a 300 magnum and an 8X57. All of which beat the 6.5 G badly for killing game, and all of which are as light and many of them lighter than those 6.5s I have been asked to make.
What I want in an "AR15 deer/antelope rifle" is "short and light" and for the short barrels carbines I find the 6.8 to have a slight edge over the 6.5.
BUT I would not feel a bit out of sorts if I was given a 6.5 Grendal with an 18" barrel to hunt with. I am sure I could and would use such a rifle for everything I'd ever carry the AR15 in 6.8 for and feel just fine with it.
If I were a young man and was only going to have one rifle for hunting game of 100 pounds to 300 pounds, and if that rifle was to be an AR15, I might choose the 6.5G