I'm waiting to hear what Jack has to say. One thing I thought I'd toss out to chew on while I'm waiting though.
Years ago, there was a PS article about this. In it, the author asked a panel of experienced long range competitors and some optical industry people for opinions. To make a long story short, there was a variety of opinions. One of the pet theories involved the changing light conditions causing an optical "shift" in the apparent target position, very much like what happens with mirage. The author came up with a way to test this theory. He solidly anchored a scope in his house with the crosshair fixed on a point a good distance away - I forget exactly how he anchored it, but it wasn't going to move. Then he checked it regularly, through a very wide variety of weather and light conditions to see what effect these conditions would have on the aim point of the scope. What he found, was that the aim point never moved, at all, basically shot the pet theory down in flames. Leaving him without an answer...
- DAA
