I was hoping to do some real shooting this morning, but storm Henry is really getting up now, and my rubbish bin has just gone flying past the window. Oh well, soon be summer
As a substitute I’ve just tried a few experiments with scope centering, although I didn’t go to the lengths of cutting one in half!
The scopes I used were a Hawke Panorama 4-12x40, an MTC Mamba Lite 3-12x40, an MTC Viper Connect 3-12x32 and a Sightron 10x42. For each I tried counting clicks, the mirror method, rotating the scope, and looking through the scope backwards (I’ll explain later).
My first and clearest finding was that for these scopes, click counting didn’t even get the scope roughly centered according to any of the other methods not even within a full turn, let alone a couple of clicks. This doesn’t mean the scopes are badly made, it’s just how they are designed: you might be lucky, and it might work for your particular scope, but it certainly didn’t work for any of mine.
Secondly, the mirror method seemed to work well, but was extremely sensitive to the exact angle of the scope on the mirror – it had to be absolutely flat and even a fraction of a millimetre off true altered the setting by many clicks. I found that I just couldn’t get consistent readings using three coins to raise the scope as suggested by tugg, the scope had to be flat on the mirror. Because of this, and because however I fiddled, I couldn’t see the image of the reticle, I couldn’t use this method for the Connect.
Provided that the scope had been accurately at 90 degrees to the mirror I found that the rotating method gave the same result as the mirror method.
While I was fiddling around I found that if you look through the scope from the wrong, ie objective end, you can clearly see the position of the lenses in the erector tube – or whatever the thing that is moved by adjusting the turrets is called. By adjusting the turrets until this was central and being very careful about the parallax I was able to get the overall centering pretty close, very easily. This worked best with the scope in the stand used for rotating, and at the right magnification - you'll soon soon see what I mean if you try it.
My conclusions so far are:
a) Counting clicks is not worth bothering with unless you have checked with one of the other methods and know it works for your particular scope.
b) As a substitute for counting the clicks, the ‘looking down the wrong end’ method is very useful for rough alignment, and seemed to work equally well for all my scopes.
c) The mirror method can give good results, but it is ultra-sensitive to angle and doesn’t seem to work on every scope.
d) The rotation method is useful for checking the results of the other methods, but is so fiddly and time consuming that I would always try the mirror first. This is probably the definitive method, although it does depend on the body of the scope being perfectly round, which it certainly should be, but….