by Tom Gaylord, a.k.a. B.B. Pelletier

Part 1

This report is for blog reader David Enoch and for several other readers who asked for it after he did. When I did the first report on the main cause of scope problems, which is the scope being adjusted either too high or too far to the right, David asked me to explain how to correct the situation.

There are several ways to correct this situation, and today I’ll explain the easiest one, which is also the one most often attempted by shooters: Shimming.

Important fact!
The problem we’re trying to correct here is that the scope does not adjust high enough to get the pellet to hit the aim point. Here’s a very important point about that. Many times, the scope will have been adjusted beyond its upper limit, and the pellet will still be striking low. So, if the shooter shims the scope like I’m about to show you, he may discover that the problem has not been fixed. That’s because the scope was adjusted way too high in the adjustment range. Even though he’s corrected the angle a little, he hasn’t corrected it far enough.

This frustrates those who are new to this sport, and they’re often put off by the fact that the fix doesn’t work. They think either their scope is trash or their gun is trash because they do not appreciate what’s really happening. I read comments all the time from people who experience this, and it’s clear to me that no one has ever explained everything to them.

I don’t know if this report will help them, either, because these people are not the type who research their problems. But if you have a friend who tells you about something like this that happened to them, you’ll now have the information to explain what’s happening.

Shimming helps
You all should know that on a gun with open sights, the rear sight has to be moved in the direction that you want the pellet to move. The rear of the scope is like the rear sight of an open-sighted gun. And the front of the scope is like the front sight, by the same logic.

If the rifle shoots too low, we want to move the rear of the scope up, and that can be done by shimming the rear scope ring. What we’re looking for is a downward angle (slanting downward toward the muzzle) to the scope, so the shim is placed under the rear scope ring to lift up the scope tube at that point. Nothing magical about it.

What is a shim?
A shim is a piece of material that causes something to move. Carpenters use shims all the time to square-up the frames of doors and windows. In their case, the shim is a wedge-shaped piece of wood they drive into a crack until the frame they’re straightening is true. Then, they break off the part of the shim that sticks out of the crack and the job is finished.

In our case, we will make shims from flat pieces of metal or plastic. We’ll cut them to the same size as the scope ring half they’re going into…or just a little smaller. Then, we’ll put them under the scope tube when we mount the scope in the rings. The shim will raise the scope tube by fractions of an inch; and if the problem isn’t too great, that’ll solve it.

cane and shears for shimming a scope
Aluminum soda cans make good shim material. I used metal shears, but good scissors (not your wife’s sewing scissors, though!) will also work well. I flatten the can in the middle to make the cut easier, and you only want one thickness of material for one shim (i.e., not both sides of the can).

shim cut and laying in scope ring
When the material is cut to size, lay it in the bottom ring. Metal will conform easily to the shape of the ring. Plastic from a soda bottle will not and will have to be flattened by the scope tube when you mount it.

Once the shim is in the bottom of the ring under the scope and the cap of the ring has been tightened, it’s very difficult to see the shim. If you can see it, it either isn’t in the ring all the way — which isn’t a huge problem — or it’s too thick, which is a problem.

How many shims?
The reason people don’t shim scopes is because too many shims will dent the scope tube when the cap screws are tightened. How many shims can safely be used depends on the thickness of the shim material. If you use metal cut from aluminum soda cans, you can use as many as two shims stacked on each other. If you use the thicker plastic cut from a 2-liter soda bottle, I would stop at just one. Any more and you risk the possibility of denting the scope tube when the cap screws are tightened.

What doesn’t work
If shimming under the rear ring will cause the rifle to shoot higher with the same sight setting, what about shimming above the rear ring (under the scope cap)? Will that push the pellet down? The answer is no. The impact point will not move if you shim above the scope.

To move the impact of the pellet down, put the shim under the front ring. That lifts the front of the scope, which is the same as moving the front sight up. Moving the front sight always moves the impact point in the opposite direction.

What about shimming left and right?
Can you shim the scope on its side to move the impact left or right? Yes, but be careful. If the shim extends down to the underside of the scope, it’ll also move the scope up or down, depending on which ring you’re shimming.

Do this first — before shimming
If you can, turn the rings or one-piece mount around and try them that way. Scope rings are not made with tremendous precision, and sometimes turning one or both of them around — and swapping the front ring with the rear ring, in the case where 2-piece mounts are being used — will move the impact point the way you want. Do this before you start shimming, as it puts less stress on the scope.

Sometimes, the design of the scope rings does not permit turning them around. This is particularly true in the case of 1-piece mounts that are also asymmetric. They can be mounted only one way and have a very limited range of positions for the scope. In this case, shimming may be the only recourse.

What about…?
I’m stopping this report here because I want to bound the information. I can talk about adjustable scope mounts, optical centering and other scope-related topics in later reports. In fact, I’ll be watching your comments to this report to see what’s needed.