Of the rifles I now have in my stable I consider 3 of them as being capable of taking elk out to my self imposed range of 500 yards, there are the 6.5 PRC with 143gr bullets, the 7mm PRC with the 175gr and on the top of the list is my 300 RUM with the 190gr ABLR which has accounted for many moose and elk in the time I have owned it, it just flat out kills. That being said I did take a beauty 6 x 6 bull in 1993 @ 542 yards with my 338 WM and a 225gr Hornady SP interlock bullet, of the 7 elk I have taken over the years the smallest caliber I used was the 7 RUSM with a 140gr core lock bullet, all those elk were dead on their feet, never had one go more than 75 yards after the hit.
My only advise is shoot the biggest caliber you can comfortably shoot good with a good bullet that will hold together, practice until you are confident you can put that bullet in the right place and you won't be disappointed, there is a huge difference between ringing a steel plate @ 500 yards off a bench than hitting an elk when the adrenaline has kicked and shooting from a field position.