How to shim a scope
By B.B. Pelletier
When a scope will not adjust far enough to get the pellet on target, some shooters shim it by adding thin material inside the scope rings or under the mounts, where they connect to the gun's receiver. Other times, your scope barely adjusts to the aim point, and you want a little adjustment room for corrections - and shimming works for that, too.
A shim is a piece of material added at a critical point to realign something. If you have ever put a folded piece of paper or cardboard under the leg of a table or chair to keep it from wobbling, you have used a shim.
Move the scope to move the pellet
Move the rear of the scope up to raise the strike of the pellet downrange. Think about it - you can't really move the front of the scope down, can you? To get a better appreciation of what I'm talking about, read the article about mounting a scope.
To make the strike of the pellet go down, raise the front of the scope. It's just the reverse of raising the rear.
The most common thing is for the pellet to strike too low. That has to do with the barrel's alignment relative to the scope base on the gun. So, most of the time the shim will be added to the rear ring or under the rear mount. But you have to use two-piece mounts to shim the rear mount. If you try to shim under the rear of a one-piece mount, you'll make it harder for the dovetail to grab the grooves on the rifle.
Shimming can bend your scope tube!
A little shimming may fix the problem, but too much can bend your scope tube! The rings are made to align with each other so the thin aluminum scope tube is held tight and straight. When one ring is shimmed higher than the other, or when one end of the scope is raised by shimming, the scope tube will be put under stress as the caps are tightened. A few thousandths of an inch may not matter, but thicker shims will cause so much misalignment that the thin scope tube will be dented when the caps are tightened.
So, if you shim, don't get carried away. Most airgunners use 35mm film as shim stock. One thickness is about all you should use, though if you don't tighten the scope ring caps too much you can get away with two thicknesses.
Adjustable mounts are better than shimming
Shimming may save time and money, but in the end adjustable scope mounts are a better solution. Several firms make them and some are more successful than others. The absolute best design is from B-Square and goes by the trade name AA adjustable scope mounts. There are many variations of AA mounts to fit specific air rifles, but they all share two things. They have gimbaled rings that cannot cause torsional stress on scope tubes, and they adjust in both directions. Most other adjustable scope mounts adjust for elevation, alone.