A 3-9 power(9X) seems a little low to me to be shooting at 500 yds. You're gonna need a rock solid rest anyways so you may as well be able to see what you're shooting at and be able to shoot at a spot on the target or animal rather than just shooting at it. I have a 4.5-14 on my 7 Mag and to me 14x is ok but I wouldn't feel over powered on magnification with a 6-18 either.
Everyone is different, but I find 3-9x perfectly adequate for big game at 500 or 600 yards. I will admit that when shooting clay pigeons at 600 yards or even for groups at 100, I prefer a 4.5-14x cranked up all the way. Big game is very different, however. With big game you have a much more visible target and it is easy to position the crosshairs in relation to the rest of the animal, even if the specific aim point is covered by the reticle.
