Probably not parallax then. You should have been close enough. I think your scope is at fault. You should need somewhere between 2.5 mils and 3.0 mils to get to 500. Needing double that tells me the reticle is not moving how you think it should be.

Were you shooting prone or off a bench? Bipod or bags? I don't know why I ask, just looking for a clue.

Not meaning to be offensive, but are you an accomplished shooter? I've seen guys at PD matches miss targets by many feet at 300-400 yards due to a lack of experience or skill.