For elk I'd go with a .54cal . You can get away with a .50cal,but you are going to rely on a big hunk of lead to drop an animal,versus higher velocity and shock with modern rifles. The maximum effective range with a ML will be around 150yds. Theres to much loss of velocity with a ML past 150yds and you have alot more bullet drop to deal with to be very accurate. Most modern in-line barrels have a fast twist.. about 1 in 32 so they shoot conicals better than round balls. the traditional ML have about 1 in 66 twist and are better suited for round balls.

I shoot a CVA in-line, it's a .54 cal and I use 365gr. conicals with 120grns of pyrodex. The CVA was 150 bucks from cabelas, it's very accurate and has a syn. stock so it's weather proof. Theres alot more maintanance with a ML cause if you load it,the easiest way to unload it is to shoot it. At that point you gotta clean it or you have serious problems with rust and corrosion.

E-mail me if you have any specific questions on muzzleloaders, I'd be glad to help.