Scope mount basics - part two
by B.B. Pelletier
Can you use 3/8" dovetail rings on an airgun?
You can but you shouldn't. Three-eighths dovetails are found on .22 rimfire rifles, and the mounts made for them are mostly very cheap. They won't stand up to the abuse of a spring rifle. They're flimsy no matter what gun they're on.
These are the rings you find in discount stores, and they are there because they are so cheap. Buyers are looking for the absolute lowest price for an item, and they don't know or care how well the rings will (or won't) work when they try to use them.
Strictly speaking, 3/8" is a little smaller than the 11mm dovetail found on airguns, so these cheap mounts will also be awkward on your gun. This is not a place to economize; buy good airgun mounts and you won't be sorry.
One piece or two?
This is a choice you must make, and I'd like to make it easier for you. Airgun scope mounts come in both one-piece mounts and two-piece mounts. One-piece mounts are somewhat more convenient to attach to most airguns, but they don't fit well on guns that have split dovetails or something blocking the dovetail, such as you'd find on the Webley Spectre.
One more thing about one-piece mounts - the rings are a fixed distance apart that cannot be changed. That can make attaching them to some compact scopes, such as the Leapers Bug Buster, impossible because the rings are too far apart.
One-inch rings or 30mm?
Most airgun scopes have a tube that measures one inch in diameter. These scopes need one-inch rings. Some of the extra-bright scopes, such as the Leapers mil-dot range-estimating scope, have a larger 30mm diameter scope tube. For these you need 30mm rings. Other than the ring size, the mounts are identical, but you absolutely cannot fit a 30mm scope into one-inch rings (or vice-versa).
Scope alignment problems
Your scope may not have enough adjustment to bring the group to the aim point. Usually, the group will be low and often to the left, as well. When this happens, you either have to shim the scope or use an adjustable scope mount.
Shimming means to put a shim or thin material under the rear scope base (if you have two-piece mounts) or between the scope and the bottom of the rear ring. This slants the scope downward and brings the group up to where it needs to be. Shim material can be thin plastic, photographic film and even metal shim stock purchased at a store for that purpose. Don't add as much as the thickness of a business card, or you'll risk bending the thin scope tube when the rings are tightened.
If you're shooting left or right, try swapping the rings front and back and even turning them around (if they are two-piece). This can sometimes help the elevation problem as well.
You may need adjustable mounts
Adjustable scope rings move up and down and left and right to perfectly align the scope without resorting to the scope adjustment knobs. They are more difficult and time-consuming to install, but you'll never run out of scope adjustments with a set of them. And, they keep your scope adjustments closer to the center of the range, where the scope performs better. Read about that in the March 24 post "Another cause of scope shift: over-adjusted scope knobs."
Mounting a scope can be challenging, but it's not beyond any of you. Selecting good mounts is a great way to begin.