Mounting Rifle Scopes

The Bushnell Professional Boresighter works very well for making sure your mounted scope is alignedwith the bore.

The Bushnell Professional Boresighter works very well for making sure your mounted scope is aligned with the bore.

In 2008 Eileen had a custom rifle built, and its first outing was a South African safari with me and the head of the rifle firm, who we’ll call Ned. She’d taken a couple animals when the scope went crazy. It was a brand-new Leupold 2.5-8×36 VX-III, and Ned started whining: “I’ve sent back a dozen brand-new Leupolds in the past couple years. That scope didn’t adjust accurately even when we test-fired the rifle.”

Luckily we had a back-up, a 1.5-5×20 Leupold I’d planned to give to my PH as a tip. Eileen started replacing the “bad” scope, while Ned hovered over her. After she’d snugged the ring screws, he tried to grab the screwdriver, saying: “Now we’ve really got to tighten those down!”

Eileen swept the screwdriver away and put in her gun case, along with the “defective” scope. After sighting-in the 1.5-5x (which adjusted accurately), she took five more animals with well-placed shots.

Back home we sent the 2.5-8x to Leupold. They couldn’t find anything wrong with it, so she put it back on the .308 and it worked fine. It’s worked like that ever since.

So what was the problem? A cynic would suspect that Leupold did fix the scope, but I found Leupolds of that era quite reliable. Instead I’m certain the problem was what Brian Bingham, a former assistant to well-known custom riflemaker D’Arcy Echols, calls “farmer tight.” When Brian went to gunsmithing school the instructors got on him for about twisting bolts and screws farmer tight, a reference to the way farmers use wrenches on tractors and other machinery. Brian resented this because he grew up on a farm, but while gunsmithing saw the results often—such as an Echols Legend rifle a customer returned after another “gunsmith” mounted the scope, with the scope crushed and its custom Echols rings bent out of shape.

I’ve encountered professionally farmer-tightened scopes on other occasions. A new Merkel K-1 single-shot rifle I purchased with a Swarovski scope already mounted started acting screwy on its first range session. I decided to replace the Swarovski with a trusted scope, whereupon the ring screws came loose with an audible crack, and the scope’s body was visibly “belted” by ring-pressure.

Soon after that a major manufacturer mounted a 2-7x Leupold on a .300 Winchester Magnum they sent me to test for an article. The scope’s magnification ring couldn’t be moved, because the mount’s rings were farmer-tight.

Between those there have been complaints from amateur scope mounters, ranging from whines about “ring marks” on scopes, to expensive scopes not working out of the box. While anything made by humans can be defective, a lot of ring marks and “bad” scopes are due to screwdriver crunching.

An average hunting scope is a relatively thin aluminum tube, with an “erector tube” inside, designed to be moved precisely to adjust the scope’s point of impact. Plus, in variable scopes, the lenses move lengthwise inside the erector tube. Over-tighten the scope rings and the exterior tube can be semi-crushed, so the scope fails to work properly—and the guy who installed it complains about how the XYZ brand rings he used left dents and scratches.

Another problem is the actions of mass-manufactured rifles sometimes aren’t perfectly machined and polished, so even correctly-machined scope mounts don’t end up level, or aligned with each other. Sometimes the rifle’s screw holes are slightly misaligned. Consequently when a scope it mounted it bends—and bends even more when the screws are farmer-tightened.

Solutions to this have been around for a while now, but many men are sure they know everything about “simple” mechanical stuff like mounting scopes. (In my experience most women aren’t nearly so sure, but that’s another story.)

The fact that a typical scope tube is slightly flexible is one reason the ring screws DON’T have to be farmer-tight: With the right amount of tightening, the tube presses back against the rings, so stays in place even on hard-kicking rifles. This is enhanced with matte-finish scopes and rings, but I once did considerable test-shooting with a Winchester Model 70 in .416 Remington Magnum using a shiny-finish Leupold in Talley steel mounts. The scope stayed in place—and did not acquire any ring-marks.

Sometimes the under-side of a mount base needs to be modified slightly to make the scope line up properly with the bore.

Sometimes the under-side of a mount base needs to be modified slightly to make the scope line up properly with the bore.

This occurred because I only used 20 inch-pounds of torque to tighten the ring-screws, the amount Talley suggests for them. For their Lightweight aluminum mounts, with integral bases, Talley recommends 15-17 inch-pounds. Not coincidentally, D’Arcy Echols tightened the 6-48 on their custom mounts to 16-17 inch-pounds (the 8-40 screws on the bases receive 35 inch-pounds).

I got to know the now-retired tech-service guy Leupold, Garth Kendig, very well over many years. After Garth phoned one day to compliment me on a couple of magazine articles praising such wimpy rounds as the .257 Roberts and 28-gauge, I asked him how tightly Leupold recommends their rings be tightened, mentioning the crushed scopes I’d seen.

“Oh, yeah,” Garth said. “We get quite a few of those in the repair shop. It’s like cartridges: Most guys figure more is better. But we recommend 28 inch-pounds on the rings, and 18 inch-pounds on the bases.

This may seem contrary to what Echols does, but isn’t. D’Arcy’s mounts use 8-40 screws on the bases and 6-48s on the rings. The Leupold mounts Garth mentioned used 6-48s on the bases and 8-40s on the rings. The tension suggestions are in the range recommended for those specific screw sizes.

Screw threads have definite mechanical limits. Over-tighten them and the screw can be ruined, which may explain why so many over-tightened scope mounts eventually come loose—and the guy who over-tightened them decided he has to tighten ‘em more.

When I first started reading articles by firearms “experts,” one common piece of advice was to really tighten scope bases screws, then place the screwdriver in the screw’s slot, whack the screwdriver’s handle with a hammer, then tighten the screw again. I tried this a few times, and still had base screws come loose. Guess why?

How much force is 15-20 inch-pounds? I can tighten a Torx-head screw to 15 inch-pounds by holding the screwdriver with the thumb and first two fingers of my right (dominant) hand. My hands are probably of average strength, so results will vary—but the point is this is far from farmer-tight, and keeps scopes firmly on most rifles.

D’Arcy Echols now supplies a pre-set torque screwdriver with their rifles, so customers can’t screw up their rings and scopes. I use a Brownells Magna-Tip Adjustable Torque handle, which can be set from 10 to 70 inch-pounds, and cost $150 when purchased. (Brownells no longer sells this driver, probably because there are so many far cheaper torque drivers available—which may or may not be accurate.)

Rather than over-torqueing the base screws, put a drop of BLUE thread-locker on each screw. Blue thread-lockers are designed to allow the screw to be backed out relatively easily, but not loosen during hunting. Do NOT use red thread-locker, which makes screws just about impossible to remove with a standard screwdriver. (Generally heat needs to be applied to red-locked screws to get ‘em loose.)

While mounting a scope it should be aligned as closely as possible with the bore, with the reticle centered. This is the highest and best use for a collimator, often commonly called a boresighter. These are often scorned by hunters who claim they can boresight a rifle without using any fancy device—but that’s true only of rifles where you can look through the rear of the bore by removing the rifle’s bolt, or with falling-block single-shots.

target practice after mounting rifle scopes.

This “tall target test,” made by adjusting the elevation turret of the scope between shots, indicates that not only is the scope mounted square to the bore, but its adjustments are consistent.

But boresighting isn’t nearly as important as making sure the scope’s reasonably aligned with the bore in the first place. If not, it won’t adjust as widely or accurately, because the inner tips of the adjustment turrets aren’t resting on the center of the erector tube—yet another possible cause for scopes being “bad” out of the box.

I’ve had great luck over the years with a Bushnell Professional Boresighter  (www.bushnell.com/scopes), which uses expandable arbors inserted into the muzzle, but in general laser collimators are less expensive and just as accurate. Some are designed for one cartridge (like Bushnell’s models) and shine the laser dot through the bore, but others magnetically attach to the muzzle, which makes them far more versatile. The most versatile I’ve used is the Wheeler Engineering Professional Laser Bore-Sighter (https://www.wheelertools.com/), which attaches to the muzzle with a magnet large enough to accommodate shotgun-slug barrels. Using either an arbor or laser collimator to make sure your scope’s lined up with the barrel prevents all sorts of problems.

Another common problem involves not getting the reticle “square” with the bore. Many hunters just eyeball this while holding the rifle at their shoulder. But many shooters tilt rifles somewhat when aiming, so the reticle ends up tilted—which causes bullets to fly in the opposite direction of the tilt, because the bore isn’t directly in line with the center of the reticle. There are several different products for eliminating scope tilt, but the best I’ve found is the Reticle-Tru from Parabola LLC (https://parabola-llc.com).

You can also check for scope tilt by shooting a “tall target” at 100 yards, with the target itself preferably a grid-type, so the reticle can be aligned with the lines in the grid. When adjusting the scope vertically up and down and shooting several shots, the bullet holes should line up parallel with the vertical lines on the target. (This is also a good test of the accuracy and repeatability of a scope’s adjustments.) But this test, of course, involves shooting quite a bit of ammunition, and mounting scopes using a collimator and Reticle-Tru will save considerable ammo-money.

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