Greetings, all. Here's a little info that I've found useful. This is something I would always do, even to brand new out of the box scopes before installing on the intended firearm. Every modern rifle or pistol scope that has internal adjustments has an optical zero point, that is, where the adjustments for the point of impact are centered with the tube, giving you the most adjustment that is possible without changing the mounts themselves. Now some mounting systems, like the Redfield type, have a dovetailed front ring and setscrews on the rear ring for coarse adjustments for wind-age (side to side).
Other mounting systems, like the weaver style or picatinny rails, don't have a way to compensate for wind-age, although some scope rings may have a small amount of adjustment possible, like the Millett rings for Weaver style bases. This is why it's very important to have the scope you are going to mount as close to optical zero as possible.
Here's a simple way to check the scope for optical zero before installation. First, you'll need to make a simple cradle that can be as easy as a shoebox, or if you mount or swap a lot of scopes, a more permanent tool. I made mine from a old miter box. the design is simple, just a frame of wood, (using metal may damage the optics finish), with a V shaped cut on either side of the framework. Make sure it is wide enough to give the adjustment turrets room to rotate without the scope being moved from the V shaped notch.
The main idea being to be able to rotate the scope 360 degrees without interference from the adjustment turrets.
Set this up on a workspace where it is stable, and put a grid type target on a stand or wall that you can see clearly through the scope.
After setting the scope on the stand, move it until the cross hairs align with the center of the grid/target. Rotate the scope on its supports without lifting or shifting it, and you may see the cross-hairs or dot move away from the center of the grid. Use the internal adjustments of the scope to correct this until you can rotate the scope 360 degrees with no shift from the center of your target or grid. The scope is now at its optical center and ready to be mounted on your weapon and zeroed for point of impact. This simple system may save you a lot of grief, as it gives you a reference point and the maximum amount of internal adjustment from the scope. This is exceptionally handy when installing used optics.
Thanks for reading! GunsmithG