Having watched elk hit the ground with everything from a .243 Win to the big Lazeronis, any caliber will work with the right bullet put in the right place. I have seen elk shot with broadheads fall quicker than many rifle hits. That said, an elk is a tough animal and can absorb a lot of energy from a bullet and still run a long ways even with solid hits.
We are so fortunate to live in such a wonderful and free country where we have so many choices in things as important as which caliber of a rifle to use for a specific animal. I am very guilty of having a "few" different rifles in a few different calibers. In ranges that are 400 yards and closer, with a good bullet most any of your modern calibers will do fine with a well placed shot.
For some reason, I have only had one or two chances at that perfect, standing broadside elk. Thusly when I head into the hills for elk, I almost always grab one of my .300 Win Mags for the slight chance that the elk that I want to shoot won't stop and stand broadside for me. From my personal experience and feelings, the 7 Mag has almost as much recoil as a .300 and for some reason, I've had to track almost every elk that I have shot or seen shot with the 7. So since you mentioned shooting out towards 600 yards and using factory ammunition, my vote goes heavily towards a .300 Win Mag.