Assuming you are open to most calibers, I would recommend 25-06, 257 Weatherby, 257 STW. or a wildcat 25 /300 WSM
(25 Pronghorn) . All of these are know for long range shooting. Any one of them is adequate for Pronghorn, mulies, and even elk. They shoot very fast bullets with flat tragetory , A 300 yard zero is not that uncommon on the wide open range.
Make a Plan and execute it.
Good Luck in 2011