I was recently asked why I shim ‘scope rings and how I fit them to ‘scope rings.
The easier answer to why shim, is to get the ‘scope as close to mechanical zero as possible. By mechanical zero I mean when the ‘scope elevation is wound as far down as possible, the rifle POI (Point of Impact) should be just below the aiming point for your chosen zero distance. So if you zero at 100m and wind the ‘scope right down to the bottom it will hit just below the target. The reason for this is two fold, one it utilises the full elevation range of the ‘scope and two, if you do have to return the rifle to zero you can just wind the ‘scope down, feel it bottom out and know you are maybe one click below zero, so much easier than having 13 clicks or whatever of redundant elevation.
How do I calculate the shim thickness? Some basic math and an xls spreadsheet, mine calculates in both metric and imperial and in MOA and mils however in simple terms one way you can calculate your own shims is as follows:
(The thickness of the shim) = (distance between rings) X (distance you want to move the point of impact at 100 yards) divided by 3600. The resulting number will be in inches.
Example: (type corrected)
Distance between rings = 4.6″
Distance to move the POI at 100 yards = 13″
Shim = 0.0166″
4.6×13/3600 = 0.0166
Great, so now we know one way of calculating shim thickness however producing and measuring a shim to three decimal places is going to be awkward at the best of times, a vernier certainly is not going to be able to measure the shim thickness accurately. I tend to aim for three decimal places eg .134″ for the last one I did, the shim should if at all possible be in one piece and a softer material such as aluminium works best for me. I drill mine (Dependant on ring type) cut or machine to size and then adjust the final thickness with a piece of wet and dry paper, pushing down on the shim with a finger and rubbing it in circles on the wet and dry on a flat surface. I use a calibrated micrometer to measure.
For my own rifles I build and fit a temporary shim that puts me within an inch or two of where I need to be, shoot and then measure how many minutes/mils below zero it is, then calculate the required increase in thickness to get to just below where I need to be. As an example a couple of my rifles have one click below true mechanical zero so I can wind the drum down until it bottoms and know I am one click below my chosen zero. It takes a couple of outings to get it right but is well worth the time if you need the full elevation range of the ‘scope.
Why don’t I try and get it right first time? The easy answer is I am trying to get to 1 click below zero which is around .004″ and given the compression of the material and other variables I find it easier to do the job in two gos.
One thing to consider is the potential stress placed on the ‘scope tube if the shim is of a significant thickness and you may wish to lap the rings or even bed the tube itself in the lower half of the rings. Alternatively you could use a set of rings with self aligning inserts.