Make the shim out of any type of soft metal or plastic sheet that is easy to cut. PUT SHIM UNDER THE REAR OF THE BASE NOT IN THE SCOPE RING.
You also need to center the reticle in the scope before bore sighting. Do this by counting the # of clicks for the full travel of either elevation or windage & divide by 2 then add that # of clicks from the maxed out position for both windage & elevation. This should center the reticle.
Use the shim (s) to get the elevation adjusted as closely as possible & the windage as close as possible using the windage adjustment screws (assuming you are using a Buehler or Redfield style base) @ 50 yds. before using the internal adjustments of the scope.
Then do the final adjustments to get the desired zero @ 100 yds using the internal adjustments on the scope for windage & elevation.
