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.
