Any of the bolt actions would be fine, the McMillan gets the nod from SOCOM in the Mk15 configuration. Although when SOCOM was testing long range sniper rifles they used a doppler radar to track the projectiles, the .50 (with standard ball ammo) starts destabilizing around 1,400 meters (the same range that the 408 Cheytac started destabilizing with its custom 419gr bullet).
I don't know if they tested Hornady's 750gr Amax in the .50, so I can't comment on stability of that bullet in the transonic zone.
If I had the money, I'd get the Barrett M82A1, because it is lighter than the others and I'm not a fan of humping heavy things any further than I have to. For accuracy I'd go McMillan.
Machine guns are awesome until you have to carry one.