All are more than adequate for elk but if you're looking for comparitive pros and cons I'll give you my opinion. I'm not real familiar with the 8mm so I'll have to leave that out. I would take the 7mm Ultra mag over the 338 because it shoots flatter and carries energy better at long ranges although the advantage isn't a whole lot. The 7mm will also have less recoil and should weigh less. The 338 will put an elk down with more authority but I still like the 7mm between the 2.