Not sure what your definition of magnum means, just no magnum in the name?
I would go about it this way. Prepare for an acceptable elk cartridge understanding that it will be more than you need for deer. For me, going with a gun with a bit less horsepower means that a larger diameter bullet should be used to make up some of the difference. This will give bigger holes and should carry more weight, helping in a number of categories.
500 yards isn't very far so many standard rounds will be in the running. The 30'06 may be pretty tough to beat here especially if you can get some of the heavier rounds out of a 24-26 inch barrel shooting well.
Scot E.