Scope Mounting: The Right Way

Hardware and farm-supply stores are my kind of places. I’m constantly on the lookout for items that will assist me when doing repair work on firearms. Most of these stores also sell sporting goods in some form or another. A lot of them sell scopes as well as mounting systems. In some cases, the high-school kid behind the counter will not only sell you a scope and mounting system, he’ll attach it to your firearm for you.

I’m inquisitive and always willing to learn new techniques, so I ask these young lads a few questions: "I see you sell scopes and mounts. Do you mount scopes here?" The answer is most often affirmative. "Can you tell me how you do that?" As you can imagine, after listening to the procedure, I usually leave the store a little weak in the knees. The Scope Is the Key Before the correct mounting system is selected, the needs of the scope must be considered. What’s the objective diameter (front end) of the scope?

Does the scope have an adjustable objective? Will the owner be using a flip-open lens cover? All of these factors add to the diameter of the front end of the scope and must be considered in the placement of the scope above the bore line. Most all scope objective diameters are measured in metric sizes’40 mm, 44 mm, and 50 mm, for example. To get the exact measurement of the scope’s objective diameter, the best method is to actually measure it with a caliper.

Divide the result by 2 and write this dimension down for future reference. If the scope has an adjustable objective, measure over the adjustment ring and then divide this dimension by 2. When objective lens covers, like the flip-open variety, become apart of the mix and area stationary part of the setup, then measure over the outside diameter of the cover and divide this by 2.

Removable lens caps are more for keeping dust off the lenses when the firearm is not in The see-through mounts were used to get the scope high, but do not serve the dual purpose they were intended for. use, so when I talk over scope setups with customers we usually decide that relieving any portion of the front scope cover that may hit the barrel is a better option than adding more height to the bases and rings.

For a scope with a one-inch tube, subtract .500 inch from the measured dimension of the objective diameter. For a 30mm tube, subtract .591 inch from the measured objective diameter. This figure was arrived at by multiplying the diameter in millimeters by .03937 to get the decimal size. The end result is the minimum height that we’ll need to get the front end of the scope up high enough so it doesn’t touch the top of the barrel.

Rings and Bases Scopes benefit the user most when mounted as close to the bore line as possible. If the scope sits inordinately high, the stock comb may smack the shooter’s cheek during recoil, and that

Above left and action will cause flinching, and at extreme heights the scope could runout of adjustment before impact zero is reached. We now have the minimum distance calculated to ensure that the scope objective doesn’t contact the barrel, so next we need to find a set of rings and bases that will be as close to that dimension as possible. Some base/ring manufacturers list nominal sizes of their low, medium, and high rings, but very few list how high the base sits above the receiver.

We need to measure that height with a caliper. The example here is a Winchester Model 70 action that has been rebarreled to .280 Remington using a #5 contour Douglas premium barrel. The owner has a Leupold 3.5-10, Vari-X III scope with an adjustable objective that he wanted mounted on the action. Which rings and bases will give the owner the lowest mounting possible without touching the barrel? Measuring the receiver ring, I came up with 1.339 inch diameter and divided that in half to get .6685.

Hold the scope over the receiver in the approximate location it will be when mounted to see where the front of the scope sits over the barrel. Measure the diameter of the barrel in that location and divide that number by 2. In this case, the halved dimension is .415 inch. Subtracting gives usa figure of .2535 inch. The diameter of the objective as measured over the adjustment ring is 2.000 inches, and that’s divided by 2 for a dimension of 1.000 inch.

Subtracting half the scope tube diameter of 1.000 inch leaves us with .500 inch. Now, and finally, we subtract .2535 inch from .500 inch and that tells us we need to come up with a base and ring combination height of .2465 inch. The customer had also brought me a set of Burris mediumheight rings and two-piece base set. With a digital caliper, I measured the height of the base from the bottom, inside radius to the top of the base at .208 inch.

The medium high, front ring measured .269 inch from the inside radius to the flat just above the dovetail, for a total of .477 inch. This would place the scope almost 1/4 inch higher than necessary. I measured a Burris low-height ring that I had in stock and found this dimension to be .154-inch high, combined with the .208-inch base, for a total height of .362 inch. This combination was still .1155-inch higher than necessary.

The closest match I found in my stock of rings and bases was with Weaver Imperial rings and bases. The base measured .192-inch high and the low front ring was .148 inch, for a total height of .340 inch. This combo will leave a gap between the scope objective and the barrel of approximately .093 inch. Setting Base Screws Some folks believe that scope-base screws should be torqued in place at 13 to 20 inch/pounds, depending on the length of the base screw.

An accurate torque wrench can run around $150, with imports costing a bit less. I don’t own a torque wrench, and I’m not a proponent of torquing #6-48 or #8-40 base screws, as they are quite small on the minor thread diameter. Torquing any screw stretches that fastener somewhat and if overdone, can twist the head off the screw. I prefer the use of a drop or two of thread-locker after the screw holes in the receiver have been cleaned of any oil and residue.

The base screws also receive a bath in a dissipating solvent that removes all traces of oil in the threads and allows the thread-locker maximum adhesion. This process has never failed me, even on bases mounted to .458 Winchester Magnum rifles. When you consider how many drops of serviceable Loc-tite there are in a bottle that costs $10, it equates to being a result-producing bargain.

For the base-mounting screws in the receiver ring, I always dry fit one screw at a time and then wiggle the base to make sure the screw hasn’t bottomed out in the blind hole. If the base feels snug against the receiver, I’ll back this screw out and then try the other screw in the same manner. If one of these screws does bottom out in the drilled-to-depth hole, I grind one or two threads off the end of the screw and then try it again until the fit is secure.

The receiver bridge usually has two through-holes for base attachment on most bolt-action rifles. Each mounting screw is snugged down individually, and then the bolt is inserted into the receiver. If no rubbing is detected on the bolt body, the fit is considered good. Before I install the

thread-locker-treated screws for the final time, I wipe a thin coat of oil on the base and then the receiver in the area the base will fit. This procedure helps prevent rusting between these two surfaces. The base screws are then tightened as much as possible with the appropriate wrench or screwdriver. There are those who give the rear end of the screwdriver a tap or two with a hammer while bottoming the screw into place, and they claim they can get an additional 1/8 to 1/4 turn of the screw into place.

I do this also, but I can’t honestly say whether I’m getting any additional movement or not. Installing the Rings The most common style of scope rings sold these days are those for the dovetail front and windage-adjustable rear bases. These are very strong systems and will serve even the heaviest of calibers. The front ring is turned into place by inserting the dovetail into the base and then rotating it 90 degrees.

You’ve heard the warnings many times and I find it amazing that anyone would use their scope to turn this ring into place. Always use either a hardwood dowel or, better yet, an aluminum or steel one-inch-diameter rod, at least 10 inches long, to turn this front ring. I began using a light smear of antiseize compound on the ring dovetail surface quite a few years ago after dry twisting the ring dovetail off a Redfield front ring.

To this day, I can’t say whether this was a flawed ring or if the interference fit was just too tight. I’ve removed rings installed by others and occasionally found the ring dovetail to show a shearing or galling affect on the engaging surface. The antiseize compound will eliminate this problem and help ease the ring into place. The bottom half of the rear ring is set into place after the windage-adjustment screws have been backed away enough to allow entry.

The goal here is to try and align the rear ring saddle with the front ring saddle as best you can. Your one-inch-diameter steel rod can now be used as a gauge to verify alignment of the ring bottoms. If alignment is difficult to determine, a light smear of yellow artist’s oil-paint on your the steel rod will help. Slide the rod back and forth a couple of times to see if the impression reveals favor on one side or the other or the rear ring to the front ring.

Adjust the rings until the front and rear ring saddles show as even a rubbing result as possible. The paint will clean off with bore solvent. Another method I use to determine whether the rings are in alignment is done with ring-alignment rods purchased from Brownells. These aluminum rods are turned to cone points and, once clamped into the scope rings, will quickly show you any mismatch of the rings.

With these rods you will be able to determine if either base needs shimming or if the rings are not parallel with the bore line. At this point, I clamp the barrel in my padded-jaw vise and install my collimator into the muzzle end of the barrel. This is done to get a view of how the scope is sitting over the bore line. If the rear ring sits too far to the left or the right, now is the time to move it to the center. If the front ring is cocked off perpendicular to the bore line it will show up also.

Don’t think for one moment that if you mount the scope tightly in the rings and then try to move the rear windage screws that the front ring will swivel along with any movement at the rear end. You will be bending the scope. And forget about those paper shims that come along with some ring sets. These can act like sponges when the rifle is used in wet conditions and will retain moisture that will rust steel rings.

Once the windage has been set as best you can with the collimator in place, take a look to see how the elevation is looking. This would be the time to shim the bottom of either the front or rear base to get the scope as close to center as possible. When the rings are tightened around the scope for the last time, we need to strive for an equal gap on both sides of the ring, and no thread-locker is needed for the ring screws. Ream or Lap?

We now have the bottom rear and front half of the rings fairly close to being in line. Next, we need to deal

with any vagaries involved with the manufacture of the rings that will prevent the scope from being held in place with maximum strength. Manufacturers allow themselves tolerances in the process to compensate for machine looseness and tooling that has become dull during cutting. The variables from their targeted dimensions can stack up to the point that the front ring may not match up with the rear ring as well as we’d like.

The situation could exist where each feature on one ring is at the high end of the tolerance and the other ring at the low end of the tolerance. Consider that the base height is also allowed a plus/minus tolerance. If these discrepancies are ignored while setting up a scope, the scope could easily be dinged, dented, or mangled.

With good scopes costing anywhere from $400 to over $1,000, I’m not going to stand in front of a customer and inform him that I hosed up a perfectly good scope because I didn’t pay attention to what I was doing during the installation process. For the most part, a decent set of rings and bases will run under $50, so the folks that make these can’t spend a whole lot of time in the manufacturing process to ensure they are perfect in every way. That’s where reaming or lapping comes into play.

An in-line ring reamer is an expensive tool to purchase, but it will remove material quickly for those instances where one ring is grossly out of alignment to the other, but I have yet to find a good set of rings that needed the kind of assistance that the ring reamer offers. I prefer the lapping process, mainly because the equipment is reasonably priced and the procedure is easy to learn. The lapping rod and compound I use was purchased from Brownells and has served me very well.

Before I start any lapping, I first check to see whether the rings really need lapping. This involves a light smear of the yellow artist’s paint on the lapping rod, but this time, with the top ring in place, snug enough that the rod will slide back and forth with some resistance. Push the rod back and forth while rotating it at least 90 degrees. If the paint has rubbed off showing a few high spots, we’ll need to remedy this condition.

Clamp the barreled action in your padded-jaw vise and protect the top surface of the barreled action with paper towels and cellophane tape to prevent any of the lapping compound dribbling into some hidden place that will cause you ulcers later. On a small piece of masking tape, I mark each ring-top half either with an "F" or an "R" and an arrow pointing toward the muzzle. This prevents confusion as to which ring belongs where. Remove the top of the rings and lay the lapping rod into the bottom rings.

Install the top rings over the rod just snugly enough to allow the rod to move fore and aft with some resistance. Smear some of the lapping compound into a few of the grooves in front of and to the rear of each ring. Work the rod until the grooves holding the lapping compound pass through each ring using a figure-eight motion. After 10 passes, remove the top ring and check your progress. If all of the bluing has been removed and no high spots are visible, go out and buy a lottery ticket.

If the bluing has only been removed in a few areas, you need to do another 10 passes. As the lapping progresses, each of the rings screws will need a slight turn to keep the rings snug against the lapping rod. Keep in mind that anything can be overdone’and that also goes for lapping. If I can get 90 percent of the rings’ internal surfaces square to one another, I’m happy and I’ll call it quits. Clean up the lapping compound and install the scope.

We now have the rings set to hold the scope over the bore line to the point where there will only be slight adjustment of the scope’s windage and elevation. Conclusions When you consider the overall cost of a rifle and scope system these days, the rings and bases assume a small cost in the equation. Yet, the importance of the rings and bases and how they contribute to the marriage of optics to the rifle are everything!

The simple task of mounting a scope on a rifle doesn’t seem so simple once the effects of poor mounting or a mangled scope become understood. I