Scopes for Airguns, part III: Mounting

There is a very wide variety of scopes mounts for airguns, differing in height, style, adjustibility and so on, but all generally fall into one of two categories: One-piece mounts, and two-piece mounts. Each has advantages and disadvantages.

one piece mount One piece mounts generally offer a wider range of adjustibility than do two piece mounts. In the pictured B-Square mount, it’s possible to adjust the verticle tilt of the scope over a very wide range- useful in some break barrel guns and in scopes with limited reticle adjustment range. These mounts are often better at resisting movement due to recoil, owing to the very large contact area between scope and dovetail. Many also feature built-in recoil stop pins designed to engage a recess machined into the top of a spring air gun.

But not every gun and scope combination has room for a one-piece mount. Loading ports, cocking levers and other mechanisms can intrude, and that’s where seperate mounting rings come in. mounting ringsSome airgunners also like the simpler, cleaner look of seperate rings, particularly on finely finished custom guns. It’s possible to buy very finely finished, polished moutning rings designed for high-end .22 rifles that will also fit a standard airgun dovetail Whichever you choose, the same general considerations involved in mounting a scope are applicable to both.

Before you do anything else, line the mounting rings with a single layer of electrical tape. This will provide cushioning, protect the tube from scratching, and help the rings grip the tube without excessive clamping force. (Some mounts come with a layer of cloth tape inside; I think the vinyl electrical tape does a better job.) Attach the rings to the scope loosely at first, so that you can find the best mounting position and identify and problems of fit.
Attach the rings to the gun- again, loosely. If you you may find at this time that the bell of the scope- the end facing the target- interferes with the barrel, particularly if it’s a very large scope. If so, you’ll have to get taller rings or a taller mount. Raise the gun to your shoulder and look through the scope. Adjust the rings and the scope fore and aft so that you can get a good scope picture- that is, you can see a full image- with your head and neck in a relaxed position.
Once you’ve found a good position for the scope, add a drop of Loctite (blue formula) to the dovetail grooves and tighten the screws that attach the rings to the dovetail. This will help tremendously in keeping the scope and mount from moving; in many instances, I’ve found that recoil stops are unecessary using the Loctite technique.

Now it’s time to level the scope. Locate a target with a long horizonal line far enough away that your scope can focus on it. This can be an actual target, a line drawn on a basement wall with the aid of a bubble level- whatever you can find in the way of an accurate reference. Next, level the gun. If you have a gun vise, that’s great, but this can also be done with a rest and a sandbag or beanbag. Place a small bubble level on a flat part of the gun, and level it. Many break barrel guns have a flat area machined at the rear of the barrel; this is a good reference point. If you can’t find a good reference point, do the best you can, and we’ll refine level when we sight in the gun.

Rotate the scope in the rings so that the target is level with the crosshair. Carefully tighten the clamping screws- going back and forth between the two rings, and alternating left and right, and constantly checking level.

Your scope is now mounted and leveled, but there’s one more task- sighting in. That’s in my next post.

Scopes for Airguns, part II: Optics

Shopping for a scope you’ll encounter a dizzying variety of optical choices- fixed, zoom, low power, 6x, 3-9×30, 8×56- how can you make sense of all this? Start by understanding the numbers and what they mean.

The first number- or number- indicate the magnification of a scope. Two numbers seperated by a dash mans that the scope is a zoom type, with variable magnification. The number on the other side of the ‘x’ is the size of the objective lens in millimeters. Thus a 2-7×32 scope has an adjustible magnification of from 2x to 7x- that is, objects look bewtween 2 and 7 times as large as they would when viewed with the naked eye- and the objective lens is 32mm in diameter. Magnification is certainly simple enough, but what does the size of the objective lens mean? How does that affect performance?

Basically, the larger the objective lens, the more light is gathered, and the brighter the image- but only up to a point. If you divide the objective lens diameter by the magnification, you get what’s called the exit pupil size- that is, the diameter of the bundle of light presented to your eye. The bigger the exit pupil, the brighter the image, until it equals the size of your eye’s pupil. Beyond that, there’s no benefit. So how big is you eye’s pupil?

It varies, depending on the brightness of the environment, and a person’s age. In bright sunlight, the pupil might contract to a 1-2mm or less; at night, a young person’s pupil might be anything from 5 to 9mm, with 7mm being around average. As we age, this decreases on the order of a millimeter or more over the course of one’s life.

Most shooting is done in daylight, and so a pupil size of more than 2-4mm is probably unecessary. Shooting around dusk- say, shooting vermin around a farm or garden- you might profitibly go as high as 7mm.

As you can see, the higher the magnification, the larger the objective lens size you need to acheive a given exit pupil. The old Beeman 2×15 SS2 scope looks tiny until you realize that it has a 7.5mm exit pupil; making it any larger would be a waste of glass. The popular 3-9×32 scopes marketed by many companies for airgun use has an exit pupil of only (32/9) = 3.5mm at maximum magnification; fine for daylight use, but too dim for use at dusk. At the other extreme, I saw a 3-9x80mm scope advertised in a British airgun shooting magazine as the ultimate night hunter- 80/9 = 8.88mm, far bigger than 99.99% of the pupils out there in the population.

Most of us need no bigger than a 4mm exit pupil for most uses, as we do most of our shooting in daylight, unlike our British cousins who sometimes hunt at night. But what about magnification? How much do you need? As always, it depends on the ranges you shoot at, and the targets you’re shooting at.

At the high end, Field Target shooters like to get as much magnification as possibkle, for two reasons- they’re shooting at some very small targets, and high magnification makes it easier to use your scope as a range finder. That’s critical in figuring out your trajectory. Most shooters seem to consider 6-18x to be a minimum range, with the use of 8-32x and 8-40x being not uncommon.
So why doesn’t everyone use a 8-40x scope? Cost is one reason, size another (they’re big!) and ease of use a third. It is extremely difficult to acquire a target at 40x- field target shooters typically acquire the target at a lower magnification, and then zoom to maximum and focus to find the range. Then they adjust their elevation and windage, and aquire the target yet again. Fine for shooting a steel target fastened to the ground, but not to good for game.

Hunters typically use lower power scopes, or even fixed scopes. I have a Burris 6x mini on my Theoben Sirocco. It’s small, very rugged, and quick and easy to use. It doens’t allow for the same precision when shooting at 55 yards as does my Simmons 6x18x scope, but I try to hunt at shorter distances than that. (A miss in Field Target means you lose a point; in hunting, it means you wound an animal, and as hunters, we should always try for quick, humane kills.) Scopes in the 2-7x and 3-9x are very popular, and indeed are probably ideal for hunting.
Plinkers, who shoot for the pure fun of it, have the widest range of choices. Some like big target scopes, others like small, simple scopes, and some like the non-magnifying scopes and optical sights commonly seen on competition handguns and combat rifles. These don’t allow the same precision in pellet placement as do magnifying scopes, but a good shooter can do well with them, and they’re the quickest and easiest to use of any sight system.

What about the leaf sights that come on just about every air rifle?  Why not just use them? Well, while they’re more than adequate for plinking, they simply don’t have the accuracy needed for airgun huting, let alone target. Sure, a lot of deer have been taken over the years with iron sights, but the lethal area on a deer- the heart and lungs- is much bigger than entire body of most of the animals airgun hunters pursue. There are always exceptions- I know of some very successful hunters who use an old Sheridan pump gun with the stock sights- but for most of us, a scope is a better choice.

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.

Cleaning Airguns

Even though they don’t leave as much residue as do firearms when fired, it’s still a good idea to periodically clean the barrel of your spring or pneumatic gun. If you’re a competitor, this goes without saying, but even plinkers and hunters can benefit from the increased accuracy that regular cleaning delivers.

When I was a competitive shooter, cleaning was generally a ritual perfomed the night before a match; these days it’s something I do when it occurs to me that I haven’t done it in a while Sometimes, with a gun that doesn’t get shot very often, that’s once a year or less. That’s okay. Unlike, say, black powder guns, where the barrel will rust away if not cleaned immediately after shooting, a neglected airgun barrel won’t decay in any way without cleaning.

So what do you need to clean your barrel? Two things: A good cleaning rod, with tips, and a cleaning product of some sort. I keep a Beeman pull-through rod in my shooting kit, where it’s available for use in the field. At home, I like a stainless steel rod, as it doesn’t less damage to a barrel than the more common aluminum rods. You’d think that aluminum, being softer than steel, would be safer, but there’s a catch: Alumnum, when exposed to air, immediately forms a protective oxide layer, and this oxide is very hard- aluminum oxide is used to make sandpaper and grinding wheels- and can actually scratch the barrel. A highly polished stainless steel rod won’t leave any marks

For a cleaning compound, do NOT use an agressive firearms cleaning solvent like Hoppes #9. It’s far more agressive than you need, and it can attack o-rings and other seals. And should it get into the compression chamber, you may find yourself buying a new spring and piston seal. I’ve been using “Break Free CLP” for many years on airguns and firearms, and I think it does a very good job. There are a good many similar products on the market, so feel free to experiment.

To clean your gun, push a few patches wetted with the CLP through the barrel- breech to muzzle, if you can- untill the patches come out clean. Then push dry patches through until they come out dry, with no sign of cleaner. There’s no need for scrubbing with bronze brushes unless you have actual leading in the barrel.

What about those cute felt pellets that you’re supposed to shoot down the barrel to clean it? Throw them away. They don’t ofer enough resistance when shot in a spring gun to prevent damage to the spring and piston seal. I suppose you might use them in a low-powered CO2 or Pneumatic gun, if you really wanted to, but a rod and patch will do a better job.

The last step is to fire your airgun a dozen times to clear out any cleaning residue and “dirty” the barrel, as target shooters say. With every shot, a little residue gets deposted in the barrel and some is scrubbed out. Equilibrium is generally reached in a dozen shots, and after that the condition of the barrel stabilizes and accuracy is maximized.