Scope Mounting for Accuracy

I started gathering information and writing this article just before hunting season here in the Northwoods of Wisconsin in 2017. Then life threw me a curve ball and kept me out of my gunsmith shop for a few years. At that time there was a rush for everyone to get their gun fixed or cleaned from last season. Missed shots was a big complaint for the year, along with the number of botched scope mountings that seemed unusually high also.

I have long lost count of the mis-mounted or ruined scopes that have come through the shop over the years. I’ve also never understood why someone would buy an expensive gun (i.e., Browning BAR) only to mount upon it an inexpensive scope (Tasco) with even cheaper rings (no-name ge- Master scope screw kit, tactical scope base conversion kit, scope shim kit, socket head screw kit. neric).

Too many gun owners will not learn how to properly mount an optic, refuse to pay a gunsmith to do it for them, use cheap scopes and rings, and then wonder why this combination doesn’t perform very well. It’s a good reason why "bad" guns get sold off after a hunt. As an example of the poor setups that have been in my shop, one shot six feet to the left and four feet low, and had no adjustment left in the scope.

To be this far off, 0.01" of alignment error at the gun is 1 MOA, so to be six feet off at 100 yards is 0.72" of rifle-to-optic misalignment, enough to be easy to see how off center the scope was to the barrel while looking down at the gun from above. Another case, a nice Nikon scope was ruined by misaligned rings that dented the barrel and broke the internal windage parts so that the reticle flopped back and forth. Nikon was nice enough to rebuild the scope for free.

Most problems with a scope holding point of aim can be tracked back Dial caliper, center punch, inchpound torque wrench, ring wrench, scope alignment rods, ring reamer, ring lapping tool, scope level, laser boresight, one-inch and/or 30mm alignment tube. to a poor job of mounting the scope. Bad ring alignment will cause accuracy-robbing stresses which pull and twist the scope and cause it to move and lose alignment. The can also cause bullet point of impact changes over time or even while sighting in the gun.

The scope should be mounted as low as possible, without the objective bell housing touching the barrel. Enough room for a scope cover plus 0.100" is ideal. Seethrough mounts, while allowing the use of the fixed sights, raise the scope up high and introduce parallax error by being too high. The bolt handle should also clear the ocular bell by 0.100" or more. The scope, rings, and bases all need to be matched not only to the gun, but also the shooter. Sometimes

higher rings may be needed to put the scope in line with the shooter’s eye. While on the subject of fitting, the length of pull will also affect this as well. Some key things are to make sure there is enough elevation and windage left in the scope to be able to cover possible ammunition and ranges that the gun will be used in. Make sure the bases are solidly mounted, rings are straight, and the scope is in line with the barrel. The reticle also needs to be level to remove inaccuracies caused by parallax error.

What I’m writing here is not the absolute gospel on mounting scopes. This is based on my experiences in mounting scopes over the years. Some of this is overkill for mounting a scope on a .22 plinker or a beater setup, however, those who shoot long distance or target competition need a solidly-mounted scope. No matter how good an optic it may be, an improperly-installed scope will never give good results. Properly installed, however, will take inaccuracy of the optics away—as in the bases, rings, and scope.

I prefer a single base with a turn-in front ring and rear windage adjustment, like the Leopold ones. If I cannot use that, a Weaver-style two piece with windage-adjustable rear rings is my second choice. My intention is to give enough information for you to successfully use this in your shop. With so much stuff on the Internet now, some people will watch a You Tube video and think that they know it all. Someday, I should thank the Internet community for keeping me in business.

Equipment and Material One cannot perform an excellent job with bad tools. I have been burned by cheap tools and usually at the time I can least afford to have the tool fail. In the first picture, this is what I use to mount a scope. While I do not endorse any one brand or company, this is what I have come to use as I know the tools will not let me down. A dial caliper to measure Top: Holes drilled for a one piece base. Notice that center screw hole is canted.

Bottom: Another Mauser with holes drilled for a two-piece base.

The rear holes are not in line with the front holes. the scope, rings, and bases, a center punch to mark the rings for maintaining proper position, a torque wrench adjustable and in inchpounds, a ring wrench for turning in scope rings safely (a wooden dowel can be used instead), scope alignment rods for checking ring alignment, a ring reamer to cut the rings straight, a ring lapping tool to lap the rings, scope level to make it easy to level the scope, a laser sight to boresight zero the scope and be on paper at 100 yards, and a oneinch or 30mm tube to check ring alignment to barrel.

Additionally, I like to have the following kits I purchased from Brownells. A Torx Scope Ring & Base Screw kit (080-924-000), Tactical Scope Base Conversion kit (080- 912-001), Scope Shim kit (080-814- 000), and Socket Head Scope Ring & Base Screw kit (080-574-000). I do not like using slotted or cross point screws for mounting and will change these out for Torx.

The Torx is easier to use a torque wrench on and doesn’t leave marks on the ring from the screwdriver scraping the edge or slipping over the head of the screw. Both area sign of sloppy workmanship and a turn off to the customer. As an alternative, hex head screws will work as well. Preliminary If the scope is mounted, take everything off: scope, rings and base(s).

I was taught in gun school to lay a three-foot straight edge across the center line of the holes for the bases, checking to see if they are inline, as well as aligned with the bore. However, there is a better way Far

I was able to slip the penny under the base. to do this check. Take some 1" long screws that fit the holes in the receiver and screw them in. There are 1" screws in both the Torx and Socket head kits. Now, look from the rear and side to see if the holes are inline, inline with the barrel, and drilled perpendicular to the top of the receiver. If everything is in alignment you are in good shape, otherwise you have to correct this issue.

If the holes are slightly off center, using bases with windage adjustment will compensate for this. Be aware of the fact that no rifle bore is exactly centered in the barrel of any gun. I have seen brand new guns from Remington, Winchester, Ruger, and others having holes drilled bad enough the bases could not be screwed on. There are a few options available to correct There is a 0.183" gap between the straight edge and the rear base. This required shims and Acraglas to make a solid rear base.

Gun removed from picture for clarity. The front ring still needs to be turned in some more. this issue. The best way is to weld up the bad holes and drill and tap them again. In doing this, the receiver will require rebluing the area of the welding and drilling, at a minimum. Another solution would be to go to a larger size screw like the 8-40, if the holes are not too far off. Mill the new holes in the correct spot and tap.

This is what I usually prefer to do, as the 8-40 is a stronger screw and does not work loose as easily as the 6-48. Quite a few manufacturers are now going to this larger size. This is also the reason I have the Tactical Scope Base Conversion kit. It has everything needed to convert the holes of the receiver and bases with. A recent project involved two different Mausers that had been done in someone’s garage or basement with a hand drill.

On one, the rear hole could not be seen in the new base and was canted to the right. Both of them required that all the holes be welded up and redrilled. Check the Scope If mounting a used scope the reticle will need to be centered. This is also true for rebuilt scopes from the factory, I once received one with the windage all the way to the left. To do this, make a jig with two V-blocks at the end of a 2×4 that is three-inches long. The end pieces need to have a 90-degree cut to hold the scope on the main tube.

This is just a simple cradle to hold the scope on. I made mine to clamp into a vise. Put the scope on the jig. While looking through it, slowly rotate the scope 180 degrees. If the center of the reticle rotates in a circle, it is not centered. Start with the up or right adjustment to center the reticle. It will take some practice but you should be able to center a scope quickly once you get the hang of it. This will also prove if you have a bad scope.

New scopes should not need this as they come centered from the factory, though it is a good practice to always check. While I have not seen this, I have

This is where lapping comes into play to correct this. heard from others of scopes left uncentered from the factory. Mount the Bases Make sure the bases fit the gun. I have one in the shop right now that has bases for the wrong gun on it and the ring elevation is off so much there were four pieces of sandpaper under the rear base to raise it up high enough. Another case was a Mauser with its receiver rear having the charging bridge ground off. I could fit a penny under the base with the front screws installed.

This required a twopiece base with shims to correct. Nothing against manufacturers of the bases, but even the best do not always fit perfectly on a gun. With a one-piece base, mount it with the front screws only and see if there is a gap between the base and the receiver. Then put in the rear only and see if there is a gap. Any gap is going to cause stresses in the base. If there is a small gap, lap the high side using 400 grit sand paper laid on the receiver.

To prevent scratch- I highlight the dimples with permanent marker to see them. Do this on one side and in the center. You will always be able to put them back where they came off. ing the receiver, tape off the edges that hang down the sides and tape those to the receiver to keep it from moving. Lap the base until there is no gap. With large gaps, shimming is the only way to correct and have a firm base. Check the screws going through the receiver do not interfere with any moving parts.

Grind these down for clearance as necessary. With two piece bases, mount the bases on the gun and then use a straight edge across the top. Check to make sure it doesn’t rock. If not straight and level, lap the high base until it is straight and doesn’t rock anymore. Also, do a visual check to make sure the bases are not canted up or down. Lay a straight edge on the front base. If there is a gap, shimming is the only way to correct this and a good reason to have the scope shim kit.

I like to bed the bases to the gun so I know there is full contact between the base and the gun. Glass bedding from Brownells works great; mix up a small batch and dye it black. Put release agent on the receiver, screws, and the holes. Put Acraglas on the base with any shims and torque down to 20 inch-pounds. While it is setting up, remove the excess. This will also hold the shims in place. Once cured, remove the base and clean up the release agent.

Once the bases are flat and level with the receiver, degrease the holes and screws. Lightly oil the top of the receiver and the bottom of the bases to prevent rusting in the future. Do not oil the screws and make sure the holes are oil free. Torque the screws down to the specifications given by the manufacture. If there aren’t any specifications, use 20-25 inch-pounds for steel bases and 15-18 inch-pounds for aluminum bases. Do not over torque as the strain will be on the screw shank and will cause it to fail.

Once the torque wrench breaks, do not be tempted to go back and retighten. You have already put enough to hold it in place. Blue thread lock can be applied if not already done by the base manufacture. Double check again to see if the screws are too long. You do not want to have to pull everything off

Notice how uneven the cut starts. This is what puts dents in scope tubes. because a screw will not let the bolt slide into the receiver. Ring Set Up The scope should be as low as possible to the barrel while allowing the shooter to bring their eye easily in line with it. With most setups, this means using medium rings for a 1" tube 3×9 scope. Midway USA, Brownells, Weaver, Leopold, and others all have charts on how to figure what rings to use. The lower the scope, the easier it is to get the elevation correct.

As a general guide, use the objective lens diameter plus the lens cover. At 20-24mm (0.787- 0.944") use LOW rings. If there is a bolt handle, ensure there is clearance. For 25-36mm (0.984-1.417") also use LOW rings while being aware the barrel may be an issue if it is heavy or thick. At 37-44mm (1.456-1.732") use MEDIUM rings, 45-50mm (1.771- 1.968") uses HIGH rings, and 51mm plus (2") uses EXTRA HIGH rings. If the customer does not have lens cov- Time to switch to the lapping rod.

This is more than enough to hold the scope through hard recoil. ers, leave room for covers so they can be put on at a later date. Installing Rings How the rings are installed goes along way in determining how the scope will perform. A sloppy job will never allow the best performance from the scope. Mount the lower half of the rings on the bases. If using turn-in rings, lightly grease the ring where it makes contact with the base before turning in.

With the rear windage adjustable base, tighten the screws to hold the rings. Then measure the top of the screw to the base with a caliper and center. Most Leopold bases will read 0.210" on both sides. With the Weaver-style bases, tighten the crossbolt to 30 inch-pounds. Put aside the screws that came with the rings and use a spare set of screws. That way you will not ding up the original screws and leave a bad looking screw head. I keep a set of working screws just for scope mounting, both base and rings.

Install the top halves of the rings and mount the scope alignment rods in them. Torque down the screws as if mounting the scope. Check the points to see how far off the rings are from being inline. With Weaver-style rings that have no windage adjustment, rotating one or both rings 180 degrees may bring the points in closer. Worst case, this may require a different set of rings if the issue is not caused by the bases being off center.

The scope alignment rod points do not have to perfectly aligned but the closer they are now, the less work later. Do not be tempted to use an alignment rod to tweak the rings to line them up. The twisting caused by doing this affects the bases and the rings and both will try to go back to original positions. With the rings in close alignment, mount a 1" aluminum tube in the rings and do a bore sight to the tube. For initial bore sighting, I use the side of a building 20 yards away for a reference.

I align the bore to the edge of the building and then look through the 1" tube. This should be close to centered in the tube. If not, move the windage over and realign the rings to bring this into center. Torque the windage screws to 45 inch-pounds. You could also use a laser bore sight to do the same thing.

Now is the time to lap the rings for good contact with the scope tube. Mark the inside center edge of the rings, top and bottom, with a punch mark. Not heavy, just enough to see the dimple. I had to highlight the dimples with permanent marker to see them. This provides a witness mark and will let you know the location and orientation of the ring halves. While not necessary, I use a scope ring reamer to make the job quicker. Install the reamer in the rings and lightly tighten down.

Keep the gap on the rings even throughout the entire lapping. Do not turn the reamer the wrong way in steel rings as it will damage it. I once had to send one in for sharpening after not paying attention and making this mistake. Turn the reamer a full turn and tighten the one screw in each ring in across pattern, only enough to tighten for drag on the reamer. Do this five times or so. Disassemble the rings and remove the reamer; there should have been some material removed by the reamer.

Note, the reamer may be removing more on one side as the rings are not perfectly aligned and therefore needs to remove where needed most. Ream until there is approximately 50% contact. Once the reamer is removing material and leaves about 50% contact in the second ring, it’s time to change to the lapping rod. Switch to using a ring lapping tool or start here if you do not have a reamer. Start with 280 grit lapping compound and put some on the top and bottom halves of the rings.

Put the lap in the rings and mount and tighten enough to cause drag as the lap is moved back and forth. This will feel gritty as it laps. Use a figure eight pattern and lap in the rings. Far Tighten the screw in across pattern and keep the gap even on both sides. When the lap moves easily, tighten 1/16 of a turn and continue. I run ten tighten cycles and then take the lap out and clean up the lap and the rings. Check to see how much material has been removed. Continue until there is around 60% to 70% contact.

Switch to 320 grit lapping compound and continue to 80% to 90% contact. This is more than enough to hold the scope and going to 100% runs the risk of going too far and losing the gap between the rings. If this does happen, use apiece of 280 to 320 grit sandpaper on a flat surface and evenly remove material on the flats of the top halves. There was a customer of mine that brought in his newly-mounted scope. He had lapped the rings to 100% contact and lost the gap to hold the scope firmly.

I told him to be back in a half hour and it would be fixed. I removed less than a 1/16" from both the top and bottom halves to restore the gap. Touched up with some Aluma-Hyde and a happy customer was out the door. Mounting the Scope Clean up the rings, especially the screws holes and remove all the lapping compound. Use a scope

308 Winchester Mauser rifle. level tool to level the scope. With the preparation done to the bases, the need to use the gun as a level should have been taken care off. Place a level across the lower ring and level the gun in the vise. Use the barrel level to match the level on the base. Using the new screws, mount the scope in the rings and leave the rings loose enough to turn the scope by hand. If the caps on the scope are flat, use them to level the scope.

Otherwise, take the cap off and use the elevation screw as a flat surface. Set the eye relief for the scope and then level it to the level on the barrel. Gently tighten the screws in a crisscross pattern while checking to make sure the scope doesn’t rotate. Tighten the ring screws to 28 inch-pounds for the Leopold. Most steel rings are 20 inchpounds while aluminum is 15 inch-pounds. The manufacturer should have a torque specifications in their instructions. Again, use blue thread lock if desired.

I double check the scope level with a plumb bob hanging on a far wall. With the gun level on the barrel level, the string of the plumb bob should be parallel with the vertical crosshair. If it is off, check the level of the ring to the barrel level. More than likely, the barrel level has slipped. Verify that the objective bell does not touch the barrel. If there are scope covers, put these on to check clearance. If the gun is a bolt action, check that the bolt has clearance to the ocular bell.

On semi-autos, put in a full mag of dummy rounds and quickly cycle the action to make sure the cases will clear the scope. You don’t want to have an upset customer come back with simple issues you could have caught in the shop. Bore Sighting With my first gun, I slammed on the scope, went out to a "range", and tried sighting it in. I wasted a lot of ammo, not to say I didn’t have fun shooting, but it would have been better if it was on the paper to begin with.

The old school method of pulling the bolt out and sighting though the bore on a distant object and adjusting the scope is no longer cost effective. A bore sight makes the job easier and quicker, along with being able to sight in guns that you can’t see through the bore due to the action. There are several methods of bore sighting, including using a collimator, cartridge-shaped lasers that chamber like ammunition, and a laser sight with adapters for the bore.

The collimator setup is only good for low-medium rings and is not adjustable for height. I also do not like putting the steel bore arbors into a barrel and find the spring clip does not always hold it steady while adjusting the scope. If you only bore sight a few calibers/cartridges, cartridge-shaped lasers work well. I do too many different calibers and chamberings to even try to buy these and then store all of them. Plus there is always that odd one that comes into the shop that they do not make a laser for.

With that said, I prefer a laser bore sight that has O-rings on it for different calibers. The O-ring will not scratch the barrel and stays put once inserted into the barrel and up to its tapered cone; this also always centers it in the bore. The only problem I have with using this is that bright days make it difficult to see the red laser dot. Since I do not have a large shop, I have to sight through a window into a storage container used to store wood for the winter.

Pointing it into the shade of the container, I can see the dot on the back wall. Carrying one of these on a hunt can also verify you are still on target if you know where the dot ends up after sighting in. A green laser is supposed to be better in day light to see but I have not popped for the cost of one of those yet. There is a 25-foot target that comes with the laser bore sight kit. Since I do not have a clear 25 feet in the shop to put up this target, I have to go outside and use my container.

Put the laser dot on the target and then adjust the crosshairs to match the target. Sighting In Now the fun begins. While no one shoots the same as another, it is good practice to make sure the gun is printing on paper. I start at 25 yards to make sure I’m definitely on the target. I want to be high and centered. Then I move out to 100 yards. Drop a shot on the target and without moving the gun, move the crosshairs to the hole. Fire a few more for effect and you should be able to wrap up the project.

When I hand over a newly-mounted scope on a gun to a customer, I have them check the eye relief and make any adjustments at that time. Use apiece of painter’s tape on the ring and scope tube to mark level before moving. Adjust eye relief and re-torque. Stress that they still need to sight in the gun to match their ammunition and how they shoot. I also give them the final target, but keep a picture for my records. AG