Originally Posted by jmden
There must be a principle at work that I'm not aware of.
Yeah, it's not like looking through a little window where the closer you are to the window anything on it gets bigger relative to what you see through it, etc. That wouldn't be very useful. It's a bit more complicated than that optically.
Collimators basically only allow through parallel light rays which is the same effect as looking at an object a very long distance away so a couple inches closer or farther makes no measurable difference. It's also how the image can be in focus and parallax free at such a short range, where any real object would not be through the scope.
Anyway, the end result is that barrel length won't matter--you won't be able to measure any difference in the size of the grid. Of course I fully recommend checking the calibration of your tools so it's certainly a good idea to check a scope you've measured "the old fashioned way" (yardstick at 100 yds or similar) to make sure the collimator's grid can be believed. Everything has manufacturing tolerances so there sure could be a +/- % on it as well. You'll find what that is, if any, by checking it against a known quantity scope.
Originally Posted by MontanaRifleman
The leupold tool is good but the slightest cant in the tool will lead to down range drifting. That's why I like to confirm with actual shooting.
The tool only has cant if you put it on crooked.
You line it up with your reticle when you put it on. Actual shooting at 50 yds (so wind will not have much effect) will work as well but it's not necessary. The tool will easily show if you have so much as a single click's worth of lateral movement for 100, 150, 200, etc, clicks of vertical movement. There's really no more accurate way to do it.