Adjustable scope mounts
by B.B. Pelletier
Yesterday, we talked about barrel droop and the fact that many airguns have it. Today, we're going to look at the solution: adjustable scope mounts. This is also the second part of an answer I'm giving to an anonymous reader who asked about an RWS C-mount.
Before we start, there have been one HECK of a lot of postings that deal with aligning airgun scopes. I've listed many of them so you can go back and read them, because the same questions seem to keep coming up. These are not all the scope postings - just the ones that deal with alignment issues.
14 What causes scope shift?
18 How to shim a scope
24 Another cause of scope shift: over-adjusted scope knobs
1 At what range should you zero your scope?
8 More about sighting-in: How to determine the two intersection points (scope-related)
5 How to optically center a scope
20 Why doesn't my gun shoot where the scope looks?
You may not need an adjustable mount
If your airgun shoots within acceptable limits and doesn't require a lot of adjustment to get on target, then you don't need an adjustable mount. Although I said yesterday that all firearms and airguns will be off as they come from the factory, many are not off by that much. This is especially true of the precharged pneumatics. The guns that seem to have the greatest problem with barrel misalignment are springers.
How to tell if you need an adjustable mount
When you have to do a lot of adjusting of either the windage or elevation knob to get the pellet to strike where you want it, you need an adjustable mount. On most scopes, the erector tube inside the scope tube is what moves the crosshairs. Opposite each adjustment knob is a coiled spring that pushes the erector tube against the adjustment knob. When the adjustment is run too far out (for example, too high for the vertical adjustment), this spring relaxes and doesn't put much tension on the erector tube. Consequently, the tube can bounce around and perhaps move to a new setting on its own accord. This is especially true on guns with heavy recoil and those with a lot of vibration. To prevent this from happening, a set of adjustable scope mounts can be installed. The scope can then be returned to the center of its adjustment range, and all necessary windage and elevation adjustments will be provided by the scope mount.
Ten years ago, there were a lot more adjustable scope mounts than there are today. Nobody had tested them to see how they worked. When writers started reporting on how adjustable mounts worked, many of them fell by the wayside because they actually damaged the scope! The RWS C-mount was one of these. The C-mount (a one-piece mount that looked like the letter "C") adjusted for elevation only. Although the rings went up and down independently at the front and rear, they didn't compensate for tilt when they weren't level. If a rigid scope tube is suddenly elevated by its rear ring and still held firmly by the front ring, the tube has to bend to accommodate the elevation difference between front and rear. The RWS C-mount was bending scope tubes!
B-Square to the rescue!
B-Square has long had an interest in airguns. The founder, Dan Bechtel, saw the problem adjustable mounts were causing and went to work on a solution. His solution was the B-Square adjustable scope mount. Not only do the rings adjust up and down independent of each other, they also tilt backward and forward, as needed, to compensate for the misalignment. When they're adjusted, they put no strain on the scope tube! They also adjust side to side.
There were several European adjustable mounts that sold for hundreds of dollars and still bent the scope tube. B-Square just about put them all out of business with their new mount. To my knowledge, only one other manufacturer made an adjustable mount that didn't bend the tube - Sportsmatch. Theirs was a one-piece mount, which proved too limiting for the American market.
I note that Gamo now has an adjustable mount. I've never examined one; from the picture, I see that it's also a one-piece and only adjusts for elevation. From the look of it, it would not strain the scope tube when adjusted. It looks simpler than the B-square rings, so if elevation is your only concern, perhaps this is a good mount for you.
Remember, you don't need an adjustable mount if you can get on target with just small adjustments of your scope. But, when you have to make large adjustments, it's time to start thinking about adjustable scope rings.