recomended whitetail and muley cartridge.

[wiredneck]

I am looking at buying a tc encore 50 cal. muzzleloader and because of the ability to change barrels im gonna buy either a 30-06 a 300 or a 308 barrel for hunting whitetails in wisconsin and also muleys in wyoming where the shots may be upwards of 500 yards. I was wondering which barrel you would recommend for flat shooting but yet small enough for the whitetail. Any other caliber sugesstions would be greatly appreciated.