I'd agree with Jeephammer that 600yds is the practical maximum for most shooters with 5.56x45. There are a few who can push it further; but even with the best shooters, you are probably looking at 800yds max.
With the budget you are discussing, you could have a Ti AR10 built in something like 6.5 Creedmoor that would be as light and accurate as a Mk12; but have considerably better hitting power and a 1,000yd practical range. Of course, you will sacrifice ammo cost somewhat (though not much if you plan on shooting Black Hills 77gr exclusively) and recoil will be a tiny bit harsher.
You also mentioned ammo commonality was important to you, so that wouldn't help there either.
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