You may get better ballistic performance out of a cartridge in the AR-10 platform, but have fun actually shooting better, consistently, with the added reciprocating mass, recoil, and gas from the AR-10 platform. Especially if you're shooting from any position besides bench or solid prone.
There are good cartridge options in the AR-15 platform to shoot at 1000 meters and beyond, especially if you reload and build with custom components. My favorite option so far; 6mm Grinch. Craddock Precision has a 6 Grinch reamer with a 0.276 neck and 0.100 freebore, which should be GTG for running magazine-fed in an AR-15 (haven't had time to install my new barrel from Craddock and work up a load yet to confirm; hopefully soon). The 'Grinch' is essentially necking down the 6.5 Grendel to 6mm and 'Dasher-izing' it. Other similar/same options are the 6mmAR Turbo40 or the 6mm Fat Rat, both supplied by guys with 'proprietary reamers'
... The first 6mmART40 upper/barrel I had pushed 105 hybrids at around 2840 fps and consistently grouped sub 1/2 moa with a 23" barrel.
Theres also the 6.5 Grendel necked down to 6mm with out blowing the shoulder out/forward, (as mentioned in a post above some options are 243 LBC, 6mm Grendel, 6mm AR, 6mm Predator). I have a 243 LBC upper with a 24" barrel. So far I've had better luck with 87 gr and 95 gr bullets than I have had with the heavies (105 - 108's) in this upper. Accuracy-wise it's nothing to write home about so far, but still shoots sub-moa. I'm pushing Berger 95gr Target VLD's at just under 3000 fps.
You might also look into the 22 Grendel. There just seems to be some accuracy 'magic' going on with Grendel case that would make me choose wildcats off of it verses something like the 22 Valkyrie. You can read a fair amount on this site about the 6 Grinch (shot from bench-rest bolt guns), being competitive against even the well known 6 PPC and 6 BR cartridges.
