6,5x284 is the last word in out to at least 1,000 yard bolt-action rifles. But this is only true in a really well built custom rifle with a 28" long top quality barrel and carefully handcrafted handloads. If you go through all this expense and effort AND learn to shoot long range (be good at reading the wind), you can be a top contender in F-class or NBRSA matches.
Now on a semi-auto platform I doubt you could get much more accuracy out of this caliber than 260 Rem, especially if you want a reasonable length barrel (22 or 24" long). The new 6.5 Creedmore would be an interesting choice, but I don't know of anyone doing this caliber in an AR10 platform....
I really think if you are wanting utmost precision you should go to a bolt-action. If you want semi-auto go to 6.5 Grendel in an AR15 action, or 260 Rem in the larger AR10 size semi-auto action.