If you cannot get enough adjustment on your rifle scope with the knobs in the turrets, then you need to know how to shim a scope. Shimming a scope means moving it slightly within the mount.
The turrets on a scope have a limited range for elevation (up and down) and windage (right and left). When you run out of room on the turret, you have to get different rings or shims or a new scope.
A shim is a temporary or a permanent solution, depending on your rifle, shooting situation and needs. It may be the best solution for various reasons. The point of shimming is to get pinpoint accuracy.
I'm Ben Baker, a lifelong hunter and shooter. I've hunted and fished across North America and Canada. I have two Safari Club International-qualifying trophies on my wall. Below, I'm going to share information about scope shims that should make you a better shooter.
Why Shim Scope Rings
Why shim anyway? Why not adjust holdover or change the mount? What is the point? A shim will raise the scope on the rear.
1) Changing the mount means going through the entire sight-in process again. At a tournament or competition, you may not have time to do this.
2) Changing mounts is no guarantee of getting the right amount of change. New mounts could be worse.
3) A shim is a lot cheaper than replace mounting and it is effective. It's sometimes necessary to overcome barrel droop in air guns.
4) Holdover is guesswork, educated guesswork, yes. If you are in a situation where you have to get a shot off quickly, you may not have time to range the target and make a holdover.
The shim stock can be just about any flexible material that fits inside the scope ring. Common materials are cutouts from plastic jugs or bottles, aluminum drink cans, brass and even clothing. Simple business cards are about as thick as you want. You can buy manufactured shims that are already cut and sized. The scope tube sits on the shim.
Credit cards are both far too thick and too stiff to use for shimming. If you need this much adjustment (see the SIZE section below), you need different rings. See the section below.
The best material also has to be thin enough to sit in the ring and still let the rifle scope tube be held firmly in place. Take a piece of sandpaper and rough the material on both sides of the shim. The rough surface helps it grip the rifle scope and the mount better than if it is smooth.
The shim should not extend past the scope rings. If it comes to the edge of the ring or almost touches the edges, that is good. Scope shims have to be thin and may need to be even thinner than what you started with. Sandpaper is a good way to reduce the material thickness.
Most of the time, the adjustment is made to the rear mount or rear ring not the front ring. A shim will give you somewhere between an inch and at most 5 inches of elevation at 100 yards. How much you get depends on the shim size and the bullet drop over 100 yards.
Two shims are needed to adjust elevation and windage.
If you need more scope change than that, it is definitely time to look into different scope mounts. If you find yourself needing to make regular shifts with the scope ring, look at a set of adjustable scope rings. These are most often used by precision, competition and tournament air rifle shooters. Barrel droop is a problem for some air gun shooters.
Once you know how much change you need, it is time to start to work with the shims.
Installation begins with cutting the shims to size. Compare the size by holding it against the ring. Do not remove the scope just yet. Once you have the rough size down, measure for thickness. Use the chart from the United Kingdom gunsmith (above) to check the thickness for the height you need. Adjust as needed.
Most scope rings are in two parts. Remove the top of the part. Leave the bottom rings attached to the rail.
Remove the scope.
Clean the mount. If your mounts do not have scoring, ridges or a roughed-up surface on the inside, hit them briefly with sandpaper to make a rough surface so your shim will stay in place. These parts are aluminum so one or twice is enough.
You can put a mild adhesive on the shim and to properly hold the two together.
Place the shim. Put the scope back in place and put the mounts back together.
Before you tighten the screws down, level the scope. Alignment is important. With the scope leveled, tighten down the screws. You should have plenty of screw length to reach through the holes. If the screws are too short, then your shim is too thick.
Sighting in the scope is necessary. You changed the position of the scope. It may not seem like much, but it is different. Again, watch the alignment.
Gently turn the knob until it is in the middle of the available range.
Now shoot and check the zero on your scope with a target. Refer to this Hunting Mark guide on sighting in your scope for help.
Here is a short video showing the correct shimming process.
Having owned, shot and sighted in many rifles over the years, I have some thoughts to share in closing.
A shim is not a defect in or the scope. Scopes and are mass-produced. If you want the most precision possible, you must make some adjustments. The maker has to produce a scope and set of mounts that function on different calibers, all of which have different bullet drop rates.
For instance, a .308 Winchester with a 168-grain boattail will drop 15-16 inches on average shooting at target at 300 yards. Depending on the bullet weight, a .45-70 Government from a carbine will drop 75-35 inches at 300 yards. Both bullet drop scenarios call for a 100-yard zero. Rifle barrel length also matters.
You can probably get by with a shim in a .308 rifle to shoot 300+ yards. In the .45-70 rifle, you are going to need a different mounting system and it may also need a shim.
The reticle does not matter.
A good shooter also knows his limits as well as the limits of his gun. No amount of adjustments will overcome the problems a poor shooter encounters down the line. Pick a gun and spend some time and bullets on the range. My friend JY Jones is a "one gun" man. His book One Man, One Rifle, One Land tracks his hunting journey across North America with one rifle to hunt many animals.