If you believe in the 1500 ft.lbs energy (which has been the "go to"on elk for decades by alot of sources) and also factor in the 2000+ '/sec minimum velocity required for some bullets to expand and explel all that energy and still hold together................. then the 6.5-284, 6.5 WSM or 264 Win Mag will easily do that at 600 yds.
IMO, the 260 and 6.5 Creedmore are kinda struggling to get there at 600 and just dont have all the poop to get er done.
Sure, without question the big 7mm's and 30 cals are better medicine for the elk and moose blues, but question is; can you shoot a 7mm or 300 something magnum good enough at 600 to capitalize on that power.......?? Can you do it without a muzzle break......??............I cant.
I personally can't imagine shooting any 7mm magnum with 160 grn bullets or heavier often enough to be really proficient without some kind of recoil reduction.......lead sled doesn't count, shooters bra's or sandbags unter the toe don't count and muzzle breaks require hearing protection.
The rules haven't changed in over 100 years of hunting, Good Shot Placement with a bullet designed for the purpose is the most important thing above all else. Just me, but I've found that the less recoil and noise: the better my shot placement. The big 6.5's are just about all that I am willing to shoot alot without extra recoil protection, and getting good at long range means alot of shootin.
Good luck in your choice and results.