A coworker wanted me to take him to the range to be sure his hunting rig was sighted in. He has a Browning A bolt in 300 win mag, with a 6x20 VXlll that has had dots installed for 2,3,4,5,6 hundred meters using load data. Anyway we shot the rifle at 100 and it shoots under a moa we started shooting farther distances and the windage starts progressively with distance going off to one side (using dots) at 500 meters windage is off several feet no wind to speak of. I shot my 7mm-08 just to see if there were winds out there that I was not seeing it shot fine. Anyway the first thing I would think is the scope was canted. At the range with eyes the cross hairs appear plumb. I would think with that much windage impact change I would defiantly see a canted cross hair. But do you think dots along a vertical cross hair are any different than changing your sight settings up and down? Any opinions out there?

Which way was the horizontal drift? If it had been just a few inches I would have suspected the rifling twist, as long as the drift was in a corresponding direction. But several feet?

At the farther ranges like that, it would not take much cant at all. Get the wheeler level set for $14 and check it. Otherwise, rifling issue, and heaven forbid, a barrel thats bowed a micrometer off. Also, maybe check the back of the bolt lugs, uneven wear may show that the bolt is offset, de-stabilizing the bullet too. Just some ideas.