The .300 RUM has some awesome ballistics. With that comes a ton of recoil. I have a friend who has one and had to get a muzzle break installed. If it were me I would stay with the tried and true .300 WM and try to get closer. I have shot 4 elk in 5 years and 125 yds is the furthest shot so seeing them is half the battle and getting close enough for a 200 yd shot or less is the rest of the story. I doubt that I have the shooting skills to even think about a 500 yd or longer shot. I know people do it but unless you are practiced and have the right gun it is a lower percentage shot than what I would take. It would be heartbreaking to see such an animal get away wounded especially if I took a low percentage shot.