I'd also say generally those with the skills for taking game at 500 yds don't need to ask questions about suitable equipment. They also would not choose a .308. If one can afford all the proposed hunts, then one could certainly afford a 300 magnum, and be much better served for the task at hand.
Truer words were never spoken! I would pick a 338 Lapua if I needed to shoot elk or bear at long range. I don't know why I would need to though? If I took a 500 yard shot I would want a good tracking dog, (if it was legal) chances of a clean kill at long distances are not good with any caliber. There are just to many things that can go wrong. (Wind gusts) Great White North
