Trade it in for an OBR;)
As far as getting the DPMS to shoot out to 1000 meters, almost all of that is barrel and if your barrel is free floated(which it is). You might want to consider just having someone rebarrel it with preferably a kreiger, or at least a fulton armory one(around 400 dollars). Why mess around with the stock sub standard barrel? You can buy a better one for your purposes instead of recrowning, cryoing etc.
Other than that reload your own ammo and you might make it. Past 800 the 308 cartridge,especially out of an autoloader is not ideal. Of course it can be done, but much easier by bolt actions and cheaper to. And significantly easier by different cartridges.
The stock DPMS is the bees knees at 600 or so. Especially considering I can buy 3 for the price of one OBR, LMT etc.