RE: Best caliber?
First check the regualations, if your state allows a .45 for Elk, they will have a minimum on the projectile weight.
In general I would say a .50 or a .54 for elk, this isn' t to say a .45 shouldn' t or couldn' t be used. A big bull can easily tip the scales at 800 pounds - so I' m inclined to say bigger is better.
Another arguement for the .50 would be rilfe and bullet selection. You' ll find a lot more options available by going with the .50