If you want a 500 yard gun that is going to require a bolt or single shot to give the accuracy needed at such extreme range. As long as you're changing guns go to a 7 Ultra mag or if you can afford the shells a 7.21 Firebird by Lazzeri. Both these two will give you a good deal more than the Rem. 7MM mag.

I shoot a Rem. 7MM mag. myself and really like it. But at 500 yards or beyond the Rem. mag. doesn't offer real strong knock down power. I've shot one deer way out there like that and you'd better be up for a tracking job. Just doesn't have the shocking ower to put them down like it does under 400 yards.