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Testing a Scope

Testing a Scope

By Ron Spomer

You can judge a scope by its looks, cost and reputation, but how do you test it for function? You really don't know how it will perform until you mount it atop a rifle and spend quality time in the mud and the blood and the rain.

Or maybe not.

There are certain tests you can perform to validate a scope before actually hunting with it. Some can be done before you even mount it on a gun. Before proceeding, make certain the scope is guaranteed so that you can return it for a full refund or replacement.

Preliminary Research
Begin your evaluation by noting the reputation of a particular brand. If you haven't heard enough about it at the gun club or from your hunting buddies, get on the Internet and search for user reviews of specific models. Brands that have been famously rugged and dependable for years might be cheapened overnight by a change in company ownership or manufacturing policy.

Also take longevity into consideration. A company that has a stellar reputation for quality and has been around for decades isn't likely to jeopardize its standing by suddenly building shoddy scopes. Nor is it likely to go out of business 10 years down the road when you crack a lens or dent a tube and
need service.

At the Shop
Compare the fit, finish and overall quality of scopes in the store. Turret caps should screw on and off smoothly and bottom against O-ring seals. Eyepieces should twist in and out readily but with enough tension to hold your chosen position. Zoom rings should turn fairly stiffly, which signifies tight tolerances and good water seals. Objective and ocular lenses should be mounted precisely in their bells without obvious crooked threads or strings of glue showing inside or out.

Shine a flashlight inside a scope or hold it under a bright light bulb to peer inside. Side walls should look clean and flat-black. Lenses should be dust-free and sit straight. There shouldn't be any bright screw heads. Look into the ocular lens held about 18 inches away. By tilting it just so, you should be able to see four or five reflections of the light source. Each represents a lens surface. Most should be tinted blue, green, yellow or red.

These coatings correct color shift and/or reduce reflections. More than one pure-white reflection suggests that not all lenses are multicoated. This isn't necessarily the kiss of death, but it can mean the scope won't transmit quite
as much light as one with fully multicoated lenses.

Eye relief is often overlooked. This is the distance between the ocular lens surface and your eye when you've attained a full field of view without seeing dark edges. Insufficient eye relief leads to painful "scope eye" when a hard-recoiling rifle drives the edge of the ocular bell into your eyebrow.

Have a buddy or store clerk measure eye-relief by holding up a ruler as you shoulder a rifle with the scope mounted. Three inches is a minimum. With a magnum, you'll be better off with 3.5 or even 4 inches. If you tend to "creep" forward on a rifle stock, you should get more eye relief than less. Shooting prone and/or at uphill angles also increases the potential for "scope eye." Be aware that many variables change eye relief with power. As power increases, eye relief shrinks. Check it all out.

Testing a ScopeAt Home
Once safely ensconced in your own laboratory, really give your purchase the torture test. Don't scratch and beat it, but test for guarantees like fogproof/waterproof. Many times, waterproof means merely water-resistant. It'll shed rain but leak if submerged. If you insist on waterproof in case you drop the scope into a creek, read the fine print before you begin these tests.

Waterproofing is accomplished by sealing joints, zoom rings, eyepiece bells, etc. with rubber gaskets. As any submariner will tell you, every gasket has a breaking point. When pressure reaches a certain point, the material - even steel - caves under it. So don't expect your scope to dive to 20 feet and come up dry.

Also realize that seals weaken over time with various changes in air and water pressure. Shipping a scope in an unpressurized baggage compartment at 35,000 feet for 15 hours puts a lot of stress on gaskets.

A minor leak might not send minnows into the breach, but anti-fogging nitrogen will leak and moisture will sneak in, fogging lenses internally. At that point, the tube might as well be filled with small fishes for all you'll see.

So check the guarantee and then double check it by sticking your scope at the bottom of a filled bucket or bathtub. Scary, isn't it? Watch for tiny bubbles, a telltale sign of a leak. Return any bubbling scopes.

If you really want to test the thing, throw it in a freezer overnight, then plunge it into a hot bath. That'll stretch and strain all the parts that expand and contract due to temperatures (hey, you will be hunting deserts at 100 degrees and the Arctic or North Dakota at below zero.) Again, bubbles mean bye-bye.

On the Range
Test optical quality by pasting a newspaper downrange and trying to read it with the sun behind and low to maximize glare. At some point, every scope will get hit with too much direct sunlight to be useful, but the better ones will maintain contrast and transmit a useable image longer than poor ones.

Another useful test checks for contrast, an important but little understood component for visual performance. In low-contrast light, objects with similar color tones tend to blend. You see this when you try to spot gray mule deer against gray sagebrush on a gray, cloudy day. A scope with excellent resolving power (ability to transmit a sharp, crisp image) and contrast will perform best.

Testing a ScopeMechanical Aptitude
Okay, so you've got a waterproof, bright, clear, contrasty scope with enough eye relief to accommodate an elephant gun. But is it accurate? Turret dials are supposed to move crosshairs specific distances, usually 1/4 or 1/2 inch at 100 yards. Does yours?

Test this by mounting it to an accurate rifle and sighting it dead-on at 100 yards. Shoot a three-shot group. Adjust the windage dial to move the point of impact 4 inches right (16 clicks of a 1⁄4-click scope, 8 clicks of a 1/2-click scope). Fire three more shots. Adjust the scope up 4 inches. Fire three more. Adjust it left 4 inches for another three-shot group. Adjust it down 4 inches and fire once more. Now draw straight right-angle lines from center to center of
each group.

Ideally the lines should create a square box, the centers of each corner corresponding perfectly to the adjustments. The last shot should have fallen within the first group of three. If the distances between groups are 4 inches but the box is leaning, the scope is not mounted square to the bore. This is your mistake, not the scope's. As long as it is moving the point of impact as indicated by the dial settings, it's true. To square it up, loosen the rings and turn the scope opposite to the lean of the box.

By the way, a squared reticle is critical for long-range accuracy because a canted reticle will result in shots being thrown at a left or right angle. The farther the target, the farther the bullet veers from point of aim along that angle. This is exacerbated if you dial corrections into a scope. Dial up three MOA in a scope reticle tilted left and you'll actually be dialing a portion of that correction left as well as up.

What if your scope reticle does not adjust accurately? You can live with such a scope if you don't depend on field adjustments to place your bullets on target. Leave the reticle locked to your sight-in distance, and it should stay there. Should you change ammunition that impacts to a significantly different point, you'll need to readjust the reticles. Just don't assume that cranking them the equivalent of 1 inch left and 2 inches lower will actually put them there. Fiddle until you get it right.

Obviously this isn't going to work with a tactical scope designed to be changed shot to shot. "Give me two MOA up and one MOA left" isn't going to fly if the scope adjustments aren't perfect and repeatable. If you value accurate and repeatable click adjustments, return any scope that doesn't measure up.

Many scopes seem to defy adjustments. You shoot a group that's 4 inches too far left, click 4 inches of supposed adjustment into the scope and your next shot is only an inch farther right instead of 4 inches. This suggests the scope is moving only a one-quarter of what it should. So you dial in another 4-inch correction, suspecting this should get you at least another inch right, but worrying that it might actually go 4 inches this time. You fire, and to your dismay find the bullet hole 8 inches right of your aim! What's going on?

Subscribe Today!Chances are the erector tube is sticking to the turret screws, then shaking loose under recoil. The way adjustments work is this: The turret screws push against the internal erector tube in which the reticle and erector lenses ride. A spring between the erector tube and bottom of the main tube constantly presses the tube up and against the turret screws. If the adjustment screws are too pointy or the erector too soft, the posts drag or stick. Manufacturers use several tactics for defeating this, including coil springs, locking springs, wider and flatter turret contact points, as well as harder erector tubes that are less likely to stick. Any and all approaches can and do work, but keep tube sticking in mind if you start having problems getting your scope adjustments to respond accurately.

Scopes aren't as easy to evaluate as a slab of salmon, but they can be tested with these methods. Best of all, most quality manufacturers guarantee them completely. Don't settle for second-rate performance.

Reprinted from the December 2005 issue of Buckmasters GunHunter Magazine. Subscribe today to have GunHunter delivered to your home.

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