As mentioned the DPMS has a good reputation for accuracy. That said I'm in the process of building a S&W M&P 10 for the same purpose; although for a little more conservative distances of between 600 - 800 meters (I plan on a 7mm-08 build to get me out to 1200 meters later; FYI this can also be done on the AR platform). Too soon to give you any worthwhile information on my M&P 10 build but if you're not in a hurry you can watch here
over the next few months as my build develops. I think the M&P 10 has a lot going for it; just too new to have any worthwhile data out there yet so I took a chance on seeing what I could get out of it. Unfortunately I don't have a 600+ yard range anywhere near me so I'm having to shoot on a friends land when I can (which right now is rare due to turkey season). In a way I'm kind of glad of the rare chances to shoot and test because it slows down my cash outlay of the mods I'm looking to test along the way. I really want to see where the money is best spent (first hand) along the way charting the benefit of each mod as I go.
All this said; I'll be watching your thread to see how you make out as well