Well, 800 meters isn't out of the spectrum of ability for an AR-10. I have shot mine out to 1,000 meters, and it can handle the task. I can assume when you say that you'd like to keep it under 2500 bucks that you are not including the cost of optics and mounting into the cost.
BMS is very much correct...the shooter's ability is the biggest factor in the long range game. Equipment is all together another issue. There are guys out there that can get behind a stock grade M1A and group a competitive national match score at 1,000 yards with open sights, and more power to them! For the rest of us that don't sport a big S on the front of our shirts and wear red underwear on the outside of our pants for no good reason, it never hurts to hit the firing line with some top-notch equipment-and therein lies the hard part, because what works for me might not work for you and vise versa.
There are some stock guns out there that are up to the task. I've never fired an OBR from LaRue, but I have shot their stealth series AR15 guns and I use a lot of LaRue peripherals on my rifle. I can say with great comfort that they make a very good product, and if Mike LaRue says his gun will hit a certain MOA at a certain range, well I am inclined to take his word for it. I am a bit skeptical of the super sass from DPMS. Again, I have never shot one. What I have seen from DPMS is a product that has been mass produced and as such the quality seems to have been sacrificed to cut down on cost. That said, the super sass might well be in a different production grade than what I've put my paws on-but I am still skeptical.
Here is an option worth looking into. I don't know if you are familiar with G A Precision guns or not, but they offer an AR10 that seems to be pretty cost effective. Once again, I have never fired their AR10 gun-but GAP builds some of the best bolt action rifles I have ever had the pleasure to fire-so much so that I paid a lot of money for not one but two of them-and both of them are around .25 MOA guns. Take a look at the testimonials on their web site-I can vouch that every praise sung to them has been truly earned. I am a sniper for the Army, and as such I keep the company of many other snipers, andwe fall into two catagories within our community; those of us that own GAP rifles and swear by them, and those of us that don't own one but want one very much so. The downfall with GAP-and this is the reason I didn't go with their AR10 when I built mine-is that you'll have to wait for several months to get one built as they are quite backed up on orders...but it would most assuredly be worth waiting for.
Yet another option, buy a stock gun, and upgrade it. For the sake of long range accuracy, I'd say the two major components to upgrade would be the barrel and the trigger group. When I built my rifle, I opted for a Lilja 24 inch heavy barrel and a JARD 2.5 lb trigger group. You will also want to put a forward hand guard on the rifle that is free floated, so as not to impart any undue torque on the barrel when assuming various firing positions. I built my rifle off of an Arma Lite platform, and I have mine pretty much customed out as far as I can possibly take it. However, as far as the accuracy of the rifle is concerned, the barrel and the trigger are without question the two most critical components that need to be addressed. You might be hard-pressed to do these modifications to an Arma Lite gun for 2500 bucks, but I think that if you play the cards a little closer to 3000 you could get it done this way.
About that .5 MOA part...It's kind of a tall order to fill. That's not to say it isn't possible, but it takes considerable tweaking to get it there. Realize, .5 MOA is a hard milestone to hit even with all but the best of bolt action rifles. When you think about all of the moving parts and mechanical actions that occur when an AR10 fires and cycles, there is a lot more going into the equation in terms of repeatable accuracy when dealing with any semi-automatic action rifle. That said and as previously mentioned, tuning an AR10 to shoot a .5 MOA consistently isn't a recipe that can be duplicated from one rifle to the next-it requires lots and lots of tweaking on the work bench, in ammo selection and/or reloading/ and in the feedback you get from the range for each individual gun. I have seen guys get lucky, buy a rifle and shoot consistently great groups right out of the box...and I'd like to slap some of them in half for it, because I've never had such luck and I'm jealous. Maybe I'm just too picky, but it usually costs me thousands of dollars and up to a year of R&D, tweaking loads, scraping and replacing components and pulling out large chunks of hair before I finally get what I'm looking for.