I have battled this issue multiple times, and have never figured this out. This video was put together by Scott with Vortex Optics. He set up a spotting scope, rigged it up in a box so it would not move at all. Set up a camera to take pics every minute of the day. It shows how your POI (point of impact) can change simply from the lighting and light angle. I have seen POI zero shift problems at 100 yards on several ranges I have shot at. I know I will need to correct and account for it. But to this day, I have not figured out HOW to account for it. I've shot rifles that shoot the same hole at 100 yards, and from morning to afternoon, have a POI shift at 100 yards. I have always liked a cloudy day for shooting and zeroing, just for this reason. It helps give you a more true trajectory path, without having to worry about the lighting and POI shifts. I thought this video was pretty cool! Here's what he said about it:
Scott w/ Vortex Optics "Over the years I have noticed funny discrepancies in my elevation on different ranges. Particularly at my home range. Even with accounting for Environmental I could have a .4 mil elevation shift at 1000 yards in less than an hour. I think we all have been at a match before and have had that one target that everybody is hitting(or missing) low or high on. Plenty of people have theories of what that is whether it's crosswind jump, atmospherics,lighting,aerodynamic jump,etc..... I still don't know exactly what causes it but what I wanted to do was catch it on film. So I set up a spotting scope with a time-lapse camera attached to it and filmed an f-class Target 1000 yards away for 9 hours(8am-5pm). I attached the spotting scope directly to the judges stand and in cased the spotting scope with a box with just a loophole for the scope to see out of. this way there would be no thermal impact on the scope or wind blowing it ,Etc...it was totally protected from the elements as I wanted to minimize any skewed outcomes. I filmed this numerous times from numerous different platforms, tripods ,judge's stand, 4 x 4 and had a similar outcome every time."
Loc: Red River way
I've noticed it being a factor depending on which direction you are shooting too. My home range faces darn near due north and it's rarely a factor there but has been noticed at matches with zero lines facing east or west.
"A vote is like a rifle; it's usefulness depends on the character of the user" Theodore Roosevelt
Loc: Parker County
"I attached the spotting scope directly to the judges stand..."
What is the judge's stand made of? Perhaps the materials that the stand is made of is expanding/contracting. I'd like to see it left in that same position and another photo taken the next day at 8am to see if it moved back to center. IMO, something is heating up during the day to cause that much movement... unless there's an active geological movement right there.
It is far more important to be able to hit the target than it is to haggle over who makes a weapon or who pulls a trigger.