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.

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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.