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When Good Scopes Go Bad

By Ron Spomer

When Good Scopes Go Bad
During a scope’s first 10 years, internal fogging shouldn’t be a problem, and thousands of scopes have gone 30 years without leaking.

One year, my custom M70 became erratic. Groups opened from .5 MOA to as much as 3 inches. I tried new bullets, new seating depths, new bedding. I could have saved wear on my barrel by checking the scope first. It wasn’t holding zero.

The 6.5-20x varmint scope had served well on a previous rifle. Why would it suddenly go berserk? “Trust me, try another scope,” gunmaker Lex Webernick of Rifles Inc. insisted. “That’s usually the problem.”

Reluctantly, I pulled the offending glass, screwed on a 3-9x and sat pop-eyed as groups shrank to sub-MOA.

Scopes are expected to shrug off temperatures from -40 to 120 degrees F; to withstand snow, rain and several pounds of water pressure in case of total submersion; to move crosshairs exactly .25 inch with each click; to zoom from 3x to 10x without shifting point of aim; to ignore air-pressure changes from sea level to 16,000 feet; and to retain reticle adjustment after being dropped on rocks. Touch off a .300 magnum, and 30 pounds of recoil slams the scope backward, stressing lenses and reticles.

That a scope can survive this year after year is amazing.

Because scopes are so durable, we take them for granted, even abuse them. This inconsiderate treatment sometimes results in problems.

Common Scope Problems

Recognizing a scope problem is half the battle. So what are the most common glitches? “Mounting systems,” says Ed Friedman, assistant product manager for Nikon Sport Optics. “You stick a $400 scope on a $2,000 rifle using a $15 set of rings and bases. Where do you see the weak link here? Too many shooters immediately blame the scope when it could just be a loose screw or a mismounted scope. This can waste a lot of time shipping back and forth.”

According to Larry Weeks of gunsmithing supplier Brownells, good hardware includes bases and rings from Leupold, Talley, Burris and Weaver. The manner in which those are mounted is also critical to consistency and holding zero.

“You can’t always trust gun stores to mount things right,” Ed Friedman says. “Sometimes they have a new kid starting out; sometimes they’re mounting dozens of scopes a day in a rush. Don’t assume everything else is perfect and blame the scope if your gun isn’t shooting right.”


“People get confused on this one,” Friedman admits. “Simply, it’s the apparent movement of the reticle over the target as you move your eye off the center-axis of the scope.”

Another way to understand parallax is to think of the scope as a camera lens that’s out of focus. Instead of focusing the image perfectly on the crosshairs, the scope focuses it slightly ahead or behind. This can result in the bullet striking 4 inches from the point of aim at 100 yards, though an inch is more common. With additional distance, this spread increases.

Look for parallax by “locking down” your rifle in a shooting rest. Center the crosshair on a tiny bull’s-eye. Shift your head side to side and up and down, watching if the bull moves away from the center reticle. If it does, that’s parallax. On a grid of 1-inch squares, you can see the number of inches of total movement.

You can focus parallax out of an adjustable-objective scope by turning the bell housing or a side-focus knob. Non-focusing scopes are preset at the factory for a specific distance, usually 100 yards for big-game scopes and 50 yards for rimfire scopes.

When Good Scopes Go Bad
An old trick for sticky tubes is to tap the turret housing several times with a screwdriver handle after making turret adjustments.

Shifting Zero

A scope that can’t hold zero wastes time, ammo and could cost you a trophy. What causes it? Usually sticky internal parts or weak springs.

The erector tube within the main tube carries the crosshairs. When you adjust windage or elevation screws, you move the erector tube, which is held under tension by one or two opposing springs. Sometimes things stick until recoil jars them loose. The upshot is confusion. Let’s say you crank 2 inches of adjustment left, fire, and see the hit has shifted only a half-inch. So you crank in another eight clicks, fire and are shocked to see the hole 4 inches left!

What happened? The tube was stuck after your first adjustment. The shot jarred it loose. You cranked in another 2 inches of adjustment and this time the freshly loosened tube moved.  So when you fired your next shot, you saw the result of a double adjustment. “This is why we always recommend shooting three shots after each adjustment,” Friedman explains. “With a single shot, you don’t know if it’s a flyer, if you pulled it badly or if the erector tube was stuck.”

An old trick for sticky tubes is to tap the turret housing several times with a screwdriver handle after making turret adjustments.

Simmons recently engineered an entirely new erector system called the TrueZero Flex Erector that does away with the erector tube spring by attaching the tube near the eyepiece, from which point it flexes slightly to accommodate reticle adjustments. There is no spring to stick or weaken. This system also will be incorporated into new Redfield scopes.

Displaced Lenses, Broken Reticles

Recoil is hard on internal parts and can jar them loose. A sharp scope suddenly gets fuzzy. Or crosshairs break.

“Any quality optic today will stand up to most recoil short of, oh, say a .416 Remington Magnum,” Friedman says. “We recently tested a Nikon with a .458 magnum, and our shooters gave up. The scope never did.”

Deer hunters should never have a problem with typical deer calibers right through the biggest .300 magnums. Low-end or discount scopes, however, are different. They’re generally engineered for recoil levels of a .30-06, sometimes a .300 Win Mag. Set them atop a light, hard-kicking rifle, and they’re likely to break loose. And recoil is cumulative. The first 100 shots might be okay, the 101st breaking the camel’s back.

The bigger the scope, the more likely it is to give way under recoil. Stick to smaller scopes on big bores.


Internal fogging results when moisture gets inside a scope and condenses on cold lenses. To prevent this, scopes are purged with nitrogen, then sealed with O-rings. The nitrogen drives out all moisture and, slightly pressurized, keeps outside air from seeping in. Eventually, however, O-rings dry and crack, nitrogen leaks out and humid air trickles in. During roughly a scope’s first 10 years, internal fogging shouldn’t be a problem, and thousands of scopes have gone 30 years with-out leaking.

External fogging occurs when warm, moist air touches a cold lens. The moisture condenses on the cold surface. The only way to prevent this is to avoid breathing on lenses, treat them with an anti-fogging solution or buy a scope with a hydrophobic exterior lens coating, such as Bushnell’s RainGuard. Leupold markets RainKote accessory lenses that screw over objective and eyepiece lenses.

Since waterproof/fogproof scopes are usually guaranteed, test them at home by placing them in a kitchen freezer overnight, then immersing them in a sink or bucket of tap water (not hot.) The idea is to mimic likely hunting conditions. Some manufacturers have told me this test is a bit unfair since “what hunter would immerse his ice-cold rifle in warm water?” True enough, but the guarantee did say “fogproof.” I’d rather discover this at home than afield. Decide for yourself what a fair test might be.

Scopes that fog internally under normal hunting conditions are a serious issue and should be returned for repair immediately.

Inconsistent Adjustments

Here’s a common problem: The reticles are supposed to move a quarter-inch at 100 yards, but you have to crank out eight of them to move your hits just 1 inch. The dials have not been accurately calibrated. This isn’t a big deal if you’re just sighting in and leaving that setting.

Inconsistent adjustments are a problem if you plan to readjust reticles afield for long-range shooting. When you click 12 times up to move the crosshair for a 400-yard shot, by golly you want it to move the specified distance. Check this the day you buy the scope, and take it back if it fails.

Today!Or course, it could start sticking later, one reason I’d rather hold over the target or use a multiple-reticle scope instead of a dial-adjust. Nonetheless, many shooters use, to complete satisfaction, dial adjustments. If you hope to be one of them, make certain your scope is consistent.

Rough Handling

Hard knocks can bump a scope off sight at the worst time. “Always, but always, check your sight-in after getting to camp, especially if you fly,” Friedman recommends. “Do this even if it means you can’t hunt the first day! Better to not even see a big buck than miss it because your rifle wasn’t sighted-in.”

Considering the conditions under which our scopes must function, they do a remarkable job, season after season. With reasonable care, they should perform flawlessly for a lifetime of hunting. Nevertheless, the prudent hunter keeps a sharp eye on them.

Reprinted from the November 2006 issue of Buckmasters GunHunter Magazine

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