Good advice on limiting the 6.5 to 500 yards, I've killed elk further with mine but won't ever do it again..
I'd go with a 300 PRC you'll have heavier bullets that are better suited to kill elk way out there.
Whatever you choose to go with make sure you're shooting a good bonded bullet.
I found out the hard way that ultra high BC is not even close to the most important thing when selecting an elk bullet.
Not to start a bullet debate but I've seen total failures on elk shoulders with 215, and 300 grain Berger's, as well as the 143 ELDX and 225 ELDM.