Scope Ring Lapping

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After mounting several hundred scopes to rifles, I have learned a few things. One of those miniature epiphanies is that manufacturing tolerances can impact how well a scope-rifle combination performs. In most cases, we are mounting a pair of bases to a receiver, then a pair of scope rings to those bases. We expect the bases to be parallel and straight, but there are several variables that can make that expectation be false. One way to fix that is to lap the rings so that they are parallel and straight.


Some 25 to 30 years ago, I tried lapping for the first time. I took a piece of 1-inch cold rolled steel round stock, lathered it up with automotive valve grinding compound and went after it in a pair of Ruger rings on a No. 1 .30-06. After a couple of minutes, I stopped to admire my progress. There was still some bluing left on the rings, so went after it some more. Minutes later, the rings were 100-percent white metal, and I thought I was done. Then I tried to cinch down the scope in the rings. With the screws down hard and tight the scope slid to and fro within the rings. I did it too much.


Recently, Brownells introduced a scope-lapping kit that, when used correctly, makes short work of ring lapping, yet produces perfect alignment of a scope within the rings. The kit includes a lapping rod that has grooves machined in it every inch or so to collect lapping compound and metal shavings, a pair of handles and an ounce of 800-grit lapping compound.


Install the lapping rod with the handles positioned conveniently in the rings with some lapping compound on it. Cover the receiver of the firearm as much as possible to prevent any compound from getting to the action. Clamp the rings tight enough to just allow the lap to move back and forth without any radial movement. A China pencil can be used to make temporary witness marks on the ring halves in order to match front and rear rings.


The instructions that come with the lap say to make 30 strokes of the lap in the rings, twisting the lap a quarter turn through each stroke and then check your work. Given my initial experience years ago, I choose to check my progress every 10 strokes or so. You’re done when you see white metal on approximately 80 percent of the inside surface of the rings. If you are lapping silver-colored rings, use a felt pen to blacken the inside of the rings to be able to check your progress. Make sure you completely clean all the lapping compound from the rings before mounting the scope. For many years, I have used powdered rosin on the inside of the rings to help grip the scope without harming its finish.


There are two alignment laps available, 1 inch and 30 mm, and they retail for $67.99 and $80.99, respectively from Brownells. If you plan on mounting more than a few scopes, this would be a worthwhile addition to your gun tools.


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1 Response to Scope Ring Lapping

Michael Van Winkle wrote:
October 15, 2011

Do you lap all of your rings when installing new rings and scope on a rifle? I had my son's 308 Ruger SS bolt action, 308 caliber, with standard Ruger Rings and Leupold VXII Scope, Rings lapped recently. This action, alone, seemed to improve the target groupings significantly. NOw I am ready to go through my very small group of rifles and have all their rings lapped? Do you think this is worth it, or should I leave well enough alone on those rifles that shoot a fairly tight group at 100 yards. I do not have any MOA accurate rifles, other than my son's Ruger 308 now.