My suggestion is a 3 part one. Number one stop trying to shoot at long range when you obviously don't have the skills for it.Number 2, the 7mmRM is a fine elk cartridge but I would suggest a heavier bullet such as what Mercy pointed out. Number 3, and this one is pretty dang important, PRACTICE MORE!!!! There is absolutely NO reason to be taking ANY shots that you have not practiced at that range. And my own little lowly opinion is there is absolutely NO reason to be taking shots past 300-400 yards PERIOD. If you can't get closer to a dang Elk than 400 yards you don't need to be hunting them anyway.
You need to be worrying less about what bullet/cartridge will have greater effect with marginal shots and worrying more about NOT MAKING THE MARGINAL SHOTS! I can't count how many poor elk and whitetail I have come across all sick and dying slow painful deaths because people didn't feel the need to practice so they could make clean and quick kills. Or because they thought because their rifle had "magnum" in the name that is was somehow magical and could make those long range shots FOR them.