RE: Best Elk Cartridge for hunting in Montana or Colorado?
Take the advice of all here who have suggested that you stay away from the big, "magnum" cartridges. I don't care what anyone says, a human simply cannot shoot them as well as they can a lower-recoiling round like the .270 Win or .30-'06. And if you happen to develop a flinch, it is most difficult to make it go away. Don't forget the .270 Win and .30-'06 have each accounted for more elk than all the new magnums combined, and they will continue to bag more each year.
Another excellent choice is the .308 Win and a few of the cartridges based on it such as the .260 Rem, 7mm-08 Rem and the .338 Federal. These numbers are extremely efficient, generate light recoil and are among the very best deer rounds available. The .338 Federal, using the 200gr Fusion or 210gr Partition, would be a terrific elk load to 300 yards or so that could also settle an argument with an angry bruin in short order.
Others that would make fine choices are the .280 Rem, .338-06 A-Square (awesome elk medicine!!!), 6.5x55 Swede, 7x57 Mauser and 8x57 Mauser.