An air or sports rifle scope sometimes reaches its maximum vertical adjustment before the rifle is sighted on the target. In many cases, you can circumvent this problem by buying different scope mounts or rings. Also, turn the elevation adjustment and count the clicks from one maximum upper to lower adjustment to make sure the reticles are properly centred across the adjustment spectrum. Shim a scope only as a last resort because it creates uneven torque on your scope that could damage it.
Determine where to shim the scope. If you need to lower the cross hairs, shim the front mount. If you need to raise the cross hairs, shim the rear mount.
Obtain shim material. It won’t take much thickness to achieve the desired elevation. Aluminium soda cans and 35mm film work well.
Unscrew the appropriate mount from the rifle’s receiver. Always shim between the receiver and the scope mount base; never inside the scope rings.
Cut the shim material with scissors or metal shears to the same size as the scope mount base. File the edges on aluminium cans.
Drill or punch holes through the shim in alignment with the scope mount screw holes.
Spray the shim, the scope mount base and the rifle receiver with a degreasing agent.
Place the shim on the receiver and attach the scope mount. Attach the scope inside the scope-mount rings.
Align the cross hairs on the target and determine whether the adjustment worked. If not, add another shim.
If two shims don’t correct the problem, try new scope mounts. Too many shims cause too much play in the scope mount and impose too much torque on the scope tube.