Elk are very tough animals. I dont know the exact reason, but they are way tougher then a black bear or moose.
The black bear and moose, relative to there size are not hard to bring down. On the other hand, a big bull elk is about twice as hard to bring down on a consistent basis. Elk very rarely give you that "1 shot drop" unless you take out both shoulders..They just plain soak up a lot of lead. I"ve had 1 shot drops with the 300 WM and 300 RUM, but they were shoulder shots...
I've used 25-06, 7mm RM, 300 WM, and 300 RUM for elk and black bears, all with success from 200-550 yards. Pick a good middle-heavy weight bullet and your good to go...
Like one of the above posters mentioned, if you wound an elk, more en likely, there going to the deepest and darkest hole (canyon) they can find. I"ve had to help track my fair share for other hunters in my camp, its not fun, trust me..