Lapping scope rings

by Tom Gaylord
Writing as B.B. Pelletier

This report covers:

  • Definition of lapping?
  • Scope ring alignment
  • Potential misalignment problems
  • The holes in the scope rings don’t line up
  • The inside of the scope rings are out of round or not finished smooth
  • Lapping scope rings
  • Check scope ring alignment
  • How are scope rings lapped?
  • Finish the job
  • Want to do it?
  • The big deal
  • Summary

This report comes by request of Pyramyd Air. It’s a subject that is not that familiar to airgunners for reasons I will explain at the end.

Definition of lapping?

Lapping is the polishing of surfaces to knock down the high spots and even-out the surface. I have addressed lapping the inner surface of a rifled barrel in the past, though I haven’t gone into it in depth. Today will be the first time I have addressed lapping scope rings.

Scope ring alignment

Scope rings are supposed to align with each other so the thin scope tube is held tight by the rings without any undue pressure resulting from misalignment. If the scope tube is perfectly round and also perfectly true (in a straight line along its entire outer surface), the holes in the scope rings need to be the same. If they are not true and also in alignment with each other, they will put uneven pressure on the scope tube when the rings are tightened.

Potential misalignment problems

There is a long list of potential scope ring misalignment issues. I will address a couple of the biggest ones.

The holes in the scope rings don’t line up

This happens more with 2-piece scope rings because their positioning on the gun is independent of each other. We presume the makers of one-piece rings take care to align them during manufacture. But two-piece rings can be out of alignment because of the scope bases they are mounted on. They can be off side to side and even up and down. It only takes a small alignment offset to create a problem.

The inside of the scope rings are out of round or not finished smooth

Cheap scope rings can have burrs and rough edges inside them that causes the scope to not fit the ring tightly. On really cheap rings it is even possible for the hole in the ring to not be round. Some are so bad they are not worth trying to fix.

Lapping scope rings

For these reasons and more many shooters have lapped the inside of their scope rings. Lapping corrects most of these problem, though if the base the rings are mounted on is the problem, lapping may not be enough. Some gunsmiths fail to take the care required to attach the scope bases to the rifle and create a problem that costs a lot more money and effort to be expended. This is far more common with firearms than airguns these days because most air rifles come with scope bases already machined into their receivers or scope tubes. 

Check scope ring alignment

To lap a set of scope rings you first mount the rings to the rifle. Then use a special pair of alignment tools that are the same diameter as the rings and taper to a point. When they are mounted in each ring with their points together, they either prove the scope ring holes are aligned or they show the misalignment.

scope ring align
Scope ring alignment tools.

ring misaligned
This is what a misaligned set of rings looks like when the tools are mounted.

If you don’t have these alignment tools you can use the lapping bar, though it will not tell you as much. A lapping bar of the same inner diameter as the rings is set in the lower ring halves. The bar looks like a scope tube. If this bar just drops into the lower rings you can proceed, but if the bar will not drop into the lower halves of the rings you must investigate why. This is where the alignment tools really pay off. 

You may discover that there is a fundamental problem that prevents proceeding. Or you could just stick the scope in the rings and try to mash it down into place by tightening the top ring caps. I have seen that done and it usually results in a dented scope tube, if not a broken scope.

How are scope rings lapped?

Scope ring lapping is grinding the inside of the scope rings to fit the outside of the scope tube. Let’s assume the lapping bar did fit down in the rings as it’s supposed to. Step one is to remove any material from inside the ring so the lapping bar can contact the ring directly. Some rings have non-slip pads inside and they must be removed.

The lapping bar is then coated with lapping compound, which is a fine grinding paste. Put the ring caps on over the lapping bar and snug them down, but not so tight that the bar can’t be moved. Now the lapping bar is both rotated and worked back and forth just a little to remove the high places on the inside of the rings. As you rotate and work the bar around, tighten the cap screws every so often, so you get an even lap. Lapping should go very quickly, but that does depend on the material from which the rings are made. Aluminum will lap much faster than steel. As you lap if you remove the bar and clean the rings you’ll see the high places that are being worn away.

Finish the job

Remove the lapping rod and clean off all the compound from the rings. Clean the lapping bar too. Then replace any material you may have removed from the rings before lapping if you really want to. A lapped set of rings will grip a scope much better, so the material may not even be necessary

Want to do it?

Lapping compound is sold in many places and is easy to find. Make sure it’s for the material your rings are made from.

A complete lapping kit can be purchased at several places online. One for both one-inch rings and 30mm rings will run about $75 and up. It includes both the alignment tools and the lapping bars in both ring sizes.

The big deal

Okay — if the rifle is an airgun and IF you buy quality rings, they probably don’t need to be lapped. Today’s scope bases and scope rings on and for airguns are very high quality. Lapping is more for the firearm user who uses two-piece rings and had to have two scope bases installed on his rifle by a gunsmith. There are so many variables there that lapping is still a viable option. But with a good set of airgun rings on a modern air rifle, lapping should be a thing of the past.

Summary

Scope lapping will never go away as long as gunsmiths attach ring bases after the gun is made. This happens a lot with older military arms. Modern firearm should come with bases that are in alignment, and the use of the Mil Std. 1913 Picatinney rail system has all but eliminated scope base issues. Combine that with a set of quality rings and the need to lap all but disappears.

47 thoughts on “Lapping scope rings

  1. BB,

    I still had your latest report on .22 reloading on my mind while reading this, and a suggestion occurred to me. Since your method is to rotate the cartridge by hand to avoid the hammer striking the already dented portion, would it help to place the brass in a tilted position while the primer slurry is still wet in order to let the primer “pool” on the side where you’re planning for the hammer to strike next? That might be easier than trying to distribute it uniformly around the entire rim. Not sure if the slurry is thin enough for gravity to work, plus air pockets might interfere, but just a thought.


  2. BB,

    Good article. Doing this for something that has never had a scope does make sense (if needed).

    The Comments RSS feed seems to (not) be working. That is what I hit first in the AM to catch up on overnight comments.

    Chris


  3. BB,

    Well, shut my mouth and fill it with corn pone. I ain’t never heard of that before!

    Seriously though, I had never heard of scope mount lapping. This could possibly explain some of the scope mounting issues I have had previously, mostly with the use of cheap rings. I have a considerable collection of scope rings, most of which I did not buy. Many times rings accompany scopes I buy. Things like that find their way to RRHFWA also.

    Recently, I discovered the fully adjustable rings.

    https://www.pyramydair.com/s/a/Sportsmatch_30mm_Rings_Fully_Adjustable_High_11mm_Dovetail/4353

    With these, there is no alignment issue. Also, if you are very careful you can center adjust your scope and adjust the mounts to almost zero. They are pricey, but when you have a nice air rifle and a nice scope…


  4. BB
    I have made remarks on a scope ring problem I found twice before but once again this seems to be another appropriate place to bring it up.
    Switching out top ring caps or installing them 180 degrees out from the way they were made may cause an alignment problem between the upper and lower half’s and transfer that alignment problem to the scope or dent it. It may be more noticeable on cheap rings but I tend to buy a lot of them for airguns.
    You can have a bunch of rings disassembled trying to figure out which ones to use and accidently switch parts between them or rotate the cap so it is not true to the bore of the lower.
    I found some so far off I could snag my fingernail on the inner surface ridge of the ring where they sit on the lower half and it disappears when I rotate it 180 degrees.
    I always check the inside alignment between the ring half’s with my finger now before installing the scope and after I get everything lined up, snug and positioned where I want it on the airgun I always tighten the scope screws first so any misalignment between the front and back rings is transferred to the picatinny rails so no damage to the scope happens. The base mounts tend to be a bit more flexible and harmed less when it comes to adapting to slight misfits.



    • Bob M

      Thanks for the tip to tighten the scope screws first. I have lots of two piece rings. Very easy to reverse or mix caps, etc. While I am not aware of having bent a scope tube yet, at least two of my popular priced scopes no longer hold zero even when turret spring is set at maximum tension. Maybe I bent something in the tube itself.

      To readers new to scope installation: Two piece scope rings exist because they often have an advantage over one piece rings in coping with eye relief and mount designs.

      Deck


  5. Entirely off topic.

    I was wasting time on YouTube and ran across a video in which the presenter was shooting a series of 4 handloads and 2 factory rounds in a 60-year-old Marlin 336 in .35 Remington chambering. The person was working up a load for the Marlin and had 4 loadings starting with a light load and moving to successively stronger loads. He was shooting on a 100-yard range and there was a target camera so that I could see the impact point on the target with each shot. Conditions were good, good light, no wind. I was astonished to see some rounds were not even on the paper. Others were fairly close to the bull but the group size was about 2 inches.

    The person then offered a short discussion regarding the need for precise crimping due to the configuration of the 336, tubular magazine, lever action. He then mentioned precision in shooting and said that after the best load was chosen he would work on accuracy in another video.

    That was a revelation. I’ve been chasing a goal of accuracy but had never considered precision as a separate matter. Huh!? As I look back on a few years of B.B. blogs, I realize that you’ve been presenting precision shooting. In fact, accuracy sits on a foundation of precision shooting skill. Once the shooter has mastered precision, the poi can be moved by adjusting the sights and sight picture to be accurate.

    I now understand that I need to choose a decent rifle and make it the best that it can be by tuning. While learning the skills needed to shoot it with precision. Only then can I expect to be able to get to accuracy. I grew up hearing of people that were “Good shots” but thought that had more to do with the gun than the marksman.

    HHmmmm, precision vs accuracy, who knew?

    It is indeed true, never too old to learn.

    Dan



    • Dan,

      As you have discovered, there is a difference between precision and accuracy. I myself am guilty of using the word accuracy when I should say precision. I suspect it is due to laziness more than anything, because if you start talking of precision you end up invariably having to explain what you mean.

      Too old to learn? I hope not. I enjoy learning something new all the time.


    • GrandpaDan,

      I have posted about Accuracy and Precision a number of times in the past few years. There is NO versus!

      https://www.mathsisfun.com/accuracy-precision.html

      (For Chris USA) Ignore the fact that MATH IS FUN is in the link!

      I have looked for a way to get this across to my grandkids and others for years! The LINK above is the best collection of ways to present to shooters given how all the different potential learning styles demand.

      I really like the links to related topics.

      Hope it helps you and lots of others.

      shootski


      • RR and Shootski,

        It seems to me that if/when I learn to control as many of the variables as are needed to shoot with precision, I can then work on making adjustments to improve accuracy. I did misstate the matter as versus. In my case, it should be a rank order, first precision, then accuracy.

        In my current situation, I’ve been buying and trying a number of rifles with a variety of power plants; one CO2, three spring guns, a nitro piston, a Crosman 120 pumper and finally a PCP. Not in that order. Still have not improved my accuracy. I think that each of those requires a different technique, to a greater or lesser degree, to achieve precision. I’ve been focused on the tool, not the one wielding it.

        Since my free time is limited for the near term. I think I’m best served by selling off most of the guns, keeping the PCP and one of the better springers. Then shoot a *lot* of pellets, learning to be a proficient shooter.

        Dan





      • BB
        Got a good laugh out of that one.
        Remember when it was common knowledge that you could keep a Harley running with bailing wire and a pair of pliers. But I must admit I never heard of anyone actually filling a flat tire with grass in a pinch.

        Think about it, in the entry below I talked about getting the proper torque in the ballpark by using the proper tool. Torque wrenches were not too common when we were one year old. Ford wrenches and pipe wrenches with wooden handles probably were and you could only get so much torque out of an 8″ wrench and it probably was right on if you did not use a section of hollow pipe over the wrench to amplify the torque. Everyone knows you only use pipe to extend a wrench to bust something loose.


        • A friend at work bought a new Honda 500 V-Twin. He called me one day saying that he had broken all the bolts on the valve covers…and he was using a torque wrench too! I agreed to go over and help him remove the broken bolts. Come to find out, he was using the bolt’s head size to determine the torque value instead of the actual bolt diameter. Live and learn as they say. I don’t think he ever did that again though.
          Geo


          • Geo791
            That’s pretty much common knowledge among mechanics and we assume everybody knows it but it’s a good thing you pointed it out. At least he tried to do the right thing, just didn’t know what it was. Following instructions in a repair manual is not an option, it’s a requirement.

            What scares me is that he actually broke all of them. Lacking that kind of situational awareness I hope he survives riding a bike.

            Bob M


            • Bob, my first bike was a ’69 BSA 441 Victor bought in early ’70. I believe the accepted method for tightening the vertically split casing to prevent oil leaks was to use an impact driver. Never had to worry about leaks with that bike. I did learn better and all my other bikes were tightened up with torque wrenches. Still wish I had that old Victor, tho.
              Larry from Algona



  6. And now those rings are dedicated to that gun.

    Once you remove them and put them back on the same gun or a different gun they will probably be off again.

    That’s why I chose to mount the scope to my Wingshot and not use a quick detachable scope ring.

    It will work for me because I probably won’t be shooting anything on the wing or running with the shot shells. And when i shoot the slugs the scope wont need resighted.

    But yep I see its something that probably needs done when mounting a scope. But that scope and rings should stay on that gun after lapping in the rings.

    And since I just mentioned the scope on my air shotgun. Now I wonder why I didn’t mount a scope on my old 410 for squirrel hunting when I was a kid. That would of been a killer combination. And thinking more I always used open sights.

    I’m thinking a scope mount or something would of needed to be made to mount a scope on a firearm shotgun. But then again maybe some had dovetails to mount a scope for slug shooting. Never paid attention.


    • Gunfun1 and Yogi,

      Thank You! You covered my thoughts on the subject.

      I read B.B.’s blog early, early, AM when no one had commented. I didn’t want to post and set the tone. I was busy this morning so got back to read the posts way late and now all I can add is:

      Yogi, you got it correct on torque!

      Thanks again,

      shootski


  7. BB
    Did you achieve that misalignment picture by simply installing the rings 180 degrees out, as noted by the position of the mount screws on each side of the dovetail? Or did you shim one for a more dramatic effect? Either way it sure makes the point clear that you need to really pay attention to details and take nothing for granted when you install a scope.
    Makes you wonder just how precise are the machines and settings used in manufacturing scope rings. Are there Monday morning rings out there?
    For example, if all the rings are bored out just a tad off to the right of center then thrown into a bin and later machined for the mounting hardware without regard to the slight offset some could be reversed for the mounting hardware effectively doubling the amount of offset when installed on the airgun,
    I don’t know how they make them but if that offset in your picture was achieved simply by flipping one mount screw to the opposite side of the rail there is a big problem with them. Or they simply assume nobody ever does that.



  8. Torque
    There is a torque value assigned to every screw and bolt simply by it’s size and some are very critical when used in certain applications. ( I’m sure general torque values can be found online ) Most are not and it’s assumed you will achieve a proper torque value if you use the proper tool to tighten them and don’t abuse them like hitting an Allen wrench with a hammer or using a 3/4″ drive breaker bar on a 1/4″ bolt. Small screws use small screwdrivers you tighten with your fingers. Just pay attention to the tightening sequence. Same with small Allen wrenches. Don’t pull on them with your arm.
    Now when it comes to stuff made in China for example you can throw everything out the window. They may use substandard material and even tightening to the proper torque may ‘screw’ things up.
    When you torque something you are actually stretching it and it wants to return to it’s original size and that creates tension to keep it tight.
    One more thing, some bolts do not return to their original size upon removal and must be discarded. Especially with high torque ones like engine head bolts. A second stretching can be too much and snap it or allow it to fail.




        • Shootski,

          But of course. Metric or SAE? Bolt material? What material is the bolt material screwing into? (the weaker metal becomes the rule). Lubricated? Flats and locks used? Bellville washers? Bolt in to inserts? Insert material? What material is the insert in? Etc..

          We had about 4 pages (around 80 listings) of commonly used applications.

          Chris



            • Shootski,

              Yup. I know what I know and don’t pretend to know more. Torque was a big thing in the electrical switchgear industry. Things come loose,.. things start to arc,.. big booms happen.

              I think that a lot of it is just common sense. The makers of products protect themselves by prescribing a torque in the event of some ham fisted idiot or someone with a wimpy wrist is on the wrench. If heat is involved,.. or heating and cooling,.. then things get more critical. Bellvilles are good for expansion situations.

              I am sure that you know much more than I on the topic.

              Chris


              • Chris USA,

                “I am sure that you know much more than I on the topic.”

                Nope, I doubt that!

                I really need to know more since I am enslaved by following the torque specifications because of my Aviation background.

                shootski


                • Shootski,

                  With aviation mechanics,… I would be anal as all heck and still be nervous as all heck. That is one field to be better safe,…than splatted into the ground from 20,000 feet.

                  Chris


        • Shootski
          For some reason I get the feeling you are trying to trap me but I’ll bite.
          Fine threads require less torque to achieve the same bolt preload. You are talking apples and oranges when it comes to comparing them. There are many different reasons for using one or the other and in general I would say fine threads are more likely to have an assigned torque value for more critical uses.

          .To answer your question I would say no. Without any specific torque value assigned you could tighten them both the same with the same wrench and torque assuming they are made of the same material and screwed into what ever they were intended for. Fine thread bolts may be longer to compensate for the strength of the finer threads.


          • Bob M,

            “For some reason I get the feeling you are trying to trap me but I’ll bite.” Nope! It was just a question I had never asked or heard discussed by knowledgeable folks. I never played GOTCHA even when I was doing Naval Air Training and Operating Procedures Standardization (NATOPS) Check Rides. I really hated the instructors/ check pilots that failed to understand the intent of training and check flights was not to make themselves feel superior.

            Thank you for the honest answer!

            shootski


            • Shootski
              Glad this is a last entry on this blog because it is way off topic.
              I went through instructor school in the Navy. It was almost mandatory as a collateral (extra) duty being an active duty reservist (TAR) training weekend warriors for 10 years.
              Later I became highly critical of civilian instructors who obviously had no formal training. I know exactly what you are talking about. Part of the training was creating test questions.

              Well at the end of a civilian DC-9 school I was in a tie with someone for No 1 in the class so the instructor gave us a few verbal test questions. “What is the main purpose of the thrust reverser hydraulic accumulator?” My competitor, ” To open the thrust reverser on the ground”. Me “To act as a store of hydraulic power for operation in the event of a hydraulic system failure and provide snubbing action for the abrupt pressure change at the end of it’s operation.
              He said I was wrong.
              I replied, “Well, I have been a Aviation Structural Hydraulics Technician for over 20 years, Came out number one in class through both formal Navy training and a two week Airforce DC-9 Hydraulics School, Supervised a DC-9 Airframes Shop for 4 years and was a Quality Assurance Inspector on it for 2 years. I’m pretty sure I got that right. Opening the thrust reversers on the ground is just a side benefit for it’s intended use without main system pressure.” “Well … we don’t teach that here”.

              I have on more than one occasion asked instructors if they were really trying to find out if the student had learned something or if they were able to figure out a misleading trick question and get it right?
              Bob M


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