Which Rifle for Antelope

[mac1]

I was invited to go on a Wyoming antelope hunt next October. I have shot dozens of coyotes in the 400 and 500 yard ranges with the .243. I dropped this buck in his tracks @300. I doubt that I would risk any shots further than this on the larger game with this rifle. Can any antelope hunters recommend a good long range rifle? I was considering a 25-06.