I think it's interesting that even though the OP specifically asked about the ideal rifle, almost all the replies including those from guys who have "been there/done that" focus on the cartridge. Even though the OP gave some specifics that would make the rifle construction probably more important than the cartridge. There are plenty of cartridges up to the task.
Open sage country and a 500 yard shot prone over a pack would point me in the direction of a rifle built for that kind of shooting. Open sage country's normally not on top of a mountain so the weight of the rifle isn't that important. Prone over a pack takes handling and balance mostly out of the equation too.
So I'd pick a rifle that's I don't own, one that's a good bit heavier than my elk rifles, with a heavy contour (5 or more) 26" barrel for the extra weight to tame recoil and extra velocity that comes with a longer barrel (don't currently own any 26's). And a suppressor so that I could stay in the scope, see the hit and see the animal's reaction. In a stock that had an adjustable cheek piece or one that's designed to be shot from prone. Topped by a good fixed 10X scope, probably an S&B PM II.
Probably in .284, but that's not as important as the rifle build.
OK....fair enough! I'd still use my M70 Mashburn.
#2 Krieger, Classic action,Echols legend Edge. 6X leupold and 7.5 pounds empty.
Similar, another Classic with Brux #2 and Borden RR stock. 6X SS. Slightly heavier than the Mashburn. Take a scale to tell.
Have been shooting both to 600 since seasons ended,and will hopefully do more. today .
If I were putting something together for that kind of elk hunting it would look and spec an awful lot like this. In fact the previously mentioned 300 Win Mag was an earlier version of this, with slightly different components.
http://www.hallowellco.com/pearce_rifle_co%20300%20Win%20Mag.htm
