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Scopes for Airguns, part I: Construction and Parallax

If you’ve been shooting a while, you’ve probably heard that not all rifle scopes are designed to handle the two-way shock of a spring piston gun. Let’s see why that’s so.

If you look at the objective end of a telescope sight- the end that faces the target- you’ll see that the objective lens is held in place with a threaded retaining ring, often sealed with a drop of lacquer or other adhesive. What’s unseen, though, is how the lens is held on the other side, or how internal elements are held. Quite often lens elements in scopes are held in place by a simple raised ring, created by rolling a die around the tube during manufacture. That’s a very accurate way of locating a lens in a scope designed for automated assembly. The problem is that it’s usually on the wrong side of the lens element to handle the shock of a piston slamming into the end of a compression chamber. The soft aluminum tubing using to make inexpesive scopes will deform as the lens cell is repeatedly pushed against it by recoil.
This style of construction is not found only in inexpensive scopes; many scopes costing $200 and up are built this way as well, and while such scopes would work fine on even magnum caliber rifles, a spring airgun can shake them loose in only a few hundred- or sometimes a few dozen- shots.

So the first thing to look for in inexpensive and moderately priced scopes is a guarentee that they’ll work in airguns- Bushnell, Simmons, and BSA (and others) sell many moderately priced scopes that come with a guarentee. Some of the very inexpensive scopes at places like Cheaper Than Dirt also come with a guarantee. The better scopes made by Burris and Leupold don’t use the rolled-ring method and can easily stand up to spring gun recoil. And of course, pneumatic guns don’t have this problem and can use any scope that will fit- so long as it’s parallax corrected for airgun ranges.

That’s another term you’ve probably come accross- “parallax corrected”. What does it mean? Close your left eye and old your hand at arm’s length so that your thumb covers some object. Now close your right eye and open your left eye. Your thumb is no longer covering the object. That’s parallax error.

In any telescopic sight, there are three basic functional units: The objective lens, which forms an image of the target; the reticle, or crosshair, which provides the aiming reference; and the ocular lens- the one you look through- which magnifies the reticle and the target image.

In a properly adjusted scope, the objective lens is focused so that the image is formed in the same plane as the resticle, and the ocular lens is adjusted so that both the reticle and the target image are in focus. Typically the ocular lens is adjustible so that individuals can fine tune it to their eyesight. The objective lens is typically not adjustible, except in high-power scopes; this is because the eye can adjust and adapt over a fairly wide range. The scope might be adjusted for, say 20 yards, but the eye can compensate enough to focus at from 10 to 50 yards.

But even though the eye might be able to compensate at 50 yards, the actual focused image will not be in the same plane as the reticle. Try this looking through a high-powered scope: Move your head slightly from side to side, and see if the position of the reticle moves relative to the target. If it does, you have parallax error. That error is enough to put you off by several inches, and that’s one of the reasons field target shooters almost all use scopes with adjustible objective focus. Low-powered scopes don’t show as much error. I have a 6x Burris Mini on a Theoben Sirocco that seesm to show alomost zero error from 10 to 55 yards. My 6-18x Simmons, though, when set for 18x, isn’t nearly as tolerant. And the 30x scopes I’ve seen used by some Field Target shooters need careful adjustment, both to minimize parallax, and just to get within the eye’s focusing tolerance.

If you do have a scope with a focusing objective, checking for parallax is a good way to make sure your focus is perfect. Although your eyes will compensate for a slightly out of focus scope, this can cause eyestrain, which isn’t ideal when you’re trying to put a pellet in a 1/4″ target at 50 yards.

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