Lapping—or truing up the inside surface of your scope rings—protects the outside of the scope tube from damage; eliminates stress on the tube that could affect the integrity of the scope's internal moving parts; and aids accuracy by removing any stresses that the scope tube could be applying to your rifle's action.
Unfortunately, few scope rings have perfectly machined surfaces where the ring contacts the scope tube. That's partly because perfectly true rings are expensive to manufacture, and your average weekend warrior isn't going to spend $130-plus on a premium set of scope rings.
Additionally, few rifle receivers are machined with absolutely true surfaces, and few of the scope-attachment screw holes where scope bases attach are drilled perfectly centered.
In short, when a ring with slightly imperfect inner surfaces is mounted to bases that are in turn screwed into slightly off-center holes in a slightly out-of-true action, it's a miracle if those inner ring surfaces turn out perfectly true.
Even proprietary type rings that mount directly to the action—such as in the case of many Ruger, Sako, CZ-USA and other rifles—are rarely perfect. In fact, for whatever reason, they're often the worst.
Not to fret: it's easy to fix.
What's harder is convincing casual shooters that it's necessary to fix. We touched on a few of the ways out-of-alignment rings with imperfect inner surfaces adversely affect your pet rifle; let's take a closer look:
Most rings aren't round where they grip the tube of that hard-earned riflescope; they often aren't even smooth. And most scopes have relatively thin aluminum tubes. Clamp an out-of-round ring with bumps and hollows firmly to that tube, and you've just introduced a new shape to your once-round scope. The divots and odd shapes affect the guts of the scope; in fact, if bad enough, they can bind up the magnification ring (or rather the moving parts that enable you to zoom in and out) and/or the gears and moving parts of the elevation and windage turrets. That's bad juju, especially if you're a precision shooter that really needs your scope to track honestly when zoomed in and out and dialed up and down for various distances.
Plus, rings that aren't lined up with each other—no matter how smooth their inner surfaces—bend the tube of your scope, further adding to the potential for bound gears and moving parts.
And finally, hard as it may be to believe, the stress that a scope tube bent against its will exerts on your rifle's action (which must hold your scope in whatever distorted position your rings put it) can have an adverse affect on accuracy. In drastic cases, it can even negate the accuracy advantages of a perfect action-bedding job.
It's worth fixing. How? By lapping—or honing, if you will—the inner surfaces of your rings with a perfectly straight, perfectly round steel bar and polishing compound.
To get started, order a lapping kit from Sinclair, Midway or Brownells. I use the Brownells Scope Ring Alignment Lap ($69.99 for the 1-inch; $79.99 for the 30mm version) and have great luck with it. Most lapping kits come with a polishing compound; if not, pick up some 800-grit compound at your local hardware store or machinist supply store.
As a side benefit, your lapping rod can be used as a ring alignment tool when turning in dovetail-type rings or whatnot.
With your rings firmly in place on your rifle, put a small piece of masking tape on the upper half of each ring and mark them "Front" and "Rear." Once lapped, you mustn't mix up your ring halves or even reverse the way they sit on the bottoms.
Take the ring screws out and the top halves off, lay a rag over your action to prevent bits of lapping compound from falling into it, and smear the inside of the rings—upper and lower halves—with polishing compound. Lay the lapping rod into the lower halves and screw the ring top halves over it. Don't tighten the ring screws, or you won't be able to move the lap.
Work the lap inside the rings, turning it and sliding it forward and back. At first, it will loosen quickly as the high points inside the rings polish off; tighten the ring screws slightly, one ring at a time, and keep working. Brownells recommends working the lap forward and back about 30 times before freshening the lapping compound; I tend to just work it for about two songs on the radio.
Unscrew the rings and apply a fresh layer of compound. Replace the rings and go back to lapping. Some fellows suggest a figure 8 pattern as you work the lap back and forth; I tend to just zigzag it forward and back when tight and run the handle in big ovals as it loosens up. Keep snugging the screws lightly as the rings loosen.
Depending on the quality of the rings and the material they're made of (steel versus aluminum), you'll need to repeat the above process two to five times. When applying fresh compound, you can wipe the old stuff out of the bottom rings with paper towel to get a visual on your progress.
About 80 percent of the lower ring surface should be smooth and perfectly true by the time you're finished. The upper rings will be a bit less. There's no need to lap until the surfaces are completely polished—in fact, you can take off too much material if you lap too much, resulting in rings that touch at the sides and no longer adequately grip the riflescope when the screws are tightened.
As you mount your scope in the lapped rings, you'll notice a significant difference. The scope will lay into them without any binding, and the top halves will fit over smoothly and the screws run in easily, coming snug all at once, not slowly as they do when un-lapped rings are distorting your scope tube or slowly bending it out of alignment.
You'll also notice how much easier it is to slide your scope forward and rearward as you finesse your eye relief to perfection, and how easy it is to rotate it slightly to level up the crosshairs even with the rings lightly snugged.
As you tighten your rings down on the scope tube, you'll be comfortable in the knowledge that you're not distorting the optic's surface, or putting a bend into it, or stressing your rifle's action and potentially introducing inaccuracies. Assuming that your scope is of reasonable quality, you'll rest easy knowing that the inner moving parts can zoom and dial without binding, too.
A stress-free riflescope is consistent, dependable and predictable. When it comes to precisely placing shots with your favorite tackdriver, those are very real advantages.
Before lapping, make sure and mark the ring top halves so you won't mix them up later. It's important to keep them oriented exactly the same throughout the lapping process and afterward.
Note the heavy texture of the inside of these rings. It's not for gripping the scope tube; it's just because it's not smoothly finished. That inner surface isn't perfectly round, either.
I like to mark the inside of my rings with a silver permanent marker, so that if I ever switch the scope in the future, I can be sure and get them back in the correct orientation.
Coat the inside of the rings with lapping/polishing compound.
Don't forget to put compound in the ring top halves, too.
Screw the ring tops on, but don't tighten the screws too snug. Work the lap in zigzags through the rings, and in figure 8s or ovals as the surfaces smooth up and the lap moves more easily. Lightly tighten the rings, one at a time, as you work.
As you can see, the inside of this ring is anything but smooth and round. The polish here is the result of one lapping session about two songs on the radio long. Apply more compound and repeat. This set of rings took four sessions to finish.
About 80 percent polish on the rings' inner surface is adequate. The upper halves will have a bit less percentage smoothed up.
With the rings lapped, this nice Leupold
mounted beautifully, without stress or distortion.