Well placed shot
its not all about the caliber its about the placement of the shot. Any Caliber 30-06 to the the .375 range with a well constructed bullet and a well placed lung shot will do the trick. Yes you do need the energy at these ranges but you can use the biggest calibers and not achieve a kill with a badly placed shot. I own a .300 win mag which is a fine rifle and would do the job no problem. 500 yards is a long shot and care should be taken when doing so. Good luck and happy hunting.