You probably want a .50 caliber. They are far and away the most popular and have the greatest variety of bullets available. The .45 caliber shoots just a little bit flatter - but seems a lot more fickle about loads. And in a number of places (like Colorado) the .50 caliber is the minimum for elk and moose.

Lots of loads to try . . . . . first choice would be a 250 gr. T/C Shockwave ahead of two 50 gr. Pyrodex pellets. If you want to "magnumize" try this same bullet with 130 gr. of Pyrodex pellets (two fifties and a thirty). Good luck.