1.Center the reticule by counting the clicks from stop to stop on both knobs and then adjusting to the half way point.
2.Sit the scope in the base, ensure the rifle, vise & base cannot move easily.
3.Draw a dark cross on a piece of paper with a straight edge; align vertically as a cross, not as an X.
4.Get a helper to move the piece of paper around on the wall until it aligns with the reticule as exactly as possible. Tape or tack the paper in that location; this is preferably at 20-30 feet from the rifle.
5.Rotate the scope in the rings while pressing down on it firmly enough to prevent any wobbling; does the reticule move off center of the cross on the paper when the scope is rotated?
6.If so, hold the scope in the most misaligned position while adjusting the knobs to correct about half the visible error.
7.Rotate back to the starting position and repeat this process turning to the left and right and adjusting the knobs as necessary until the reticule remains exactly aligned on the center of the cross on the paper as the scope is rotated.
8.Now you know the reticule is centered in the scope tube, or to be more exact, in the optical axis of the scope.
9.This would be the time to center the adjustments on the drums,
without moving the reticule, so that those adjustments reflect the true center of adjustment. Depending on how much declination you want in the vertical adjustments, you may want to center only the windage adjustment at this point.
If you cannot get the reticule to center you either have mechanical play in what is holding the rifle, the attachment of the base to the rifle, or in your rotation of the scope. Or you have a scope with a parallax problem; with the scope held rigidly move your eye left and right and up and down past the field of view/exit pupil. If the reticule moves the scope is out of adjustment for parallax, at least at that range.
Now that you have the scope reticule properly collimated to its tube, take a fired case for this rifle and drill a hole in the center of the spent primer. If you're using a Boxer primed case, just drill out the primer with a 3/16" or so drill just deep enough to expose the priming hole.
1. Insert the drilled out case in the chamber. If it won't sit in place, wrap it with one run of Scotch tape, and cut the tape with a razor blade and peel off any overlap.
2. Sight through the primer/drilled hole and move the same piece of paper until the cross aligns with the bore.
3. Sit the scope in the open mount base again and look through while rotating it in the mounts; does the reticule align with the vertical line of the cross? If it doesn't your rings are not aligned horizontally and the problem is either in the alignment of the mount axis to the mount base, or the alignment of the receiver holes to the rifle bore, or if you're really unlucky, both!
The problem is much more likely to be the mount than the integral holes or dovetails on the rifle receiver.
That's how I would do it, but always open to better ideas.
