Forget all of the magazine article magnums. I see the flatlanders show up every year packing them and the locals using their same old rifles. You wont go wrong if you have something in the 308 - 30/06 - 8x57 class, even a 6.5 Swede will work if you do your job.
My buddy was guiding one of these guys this year. He was shooting a 300 RUM with a light bullet so he could get some speed. He shot 5 times and only connected with the first which disentegrated on the shoulder.