April 15, 2022
Modern machining is great, but it’s not the cure-all that many would suggest. Products are made to a specification or spec, and within that spec is a tolerance, basically a margin for error. What does this have to do with mounting a scope? Well, think of it this way. Your rifle’s receiver was made by one manufacturer, to a certain set of specs and tolerances. Your scope rings and mounts were likely produced in a different facility by a different company, to its own set of specs and tolerances. Finally, your scope is likely made by yet another party. The fact is, it is a wonder that any of these products match up as well as they do.
Why does this matter? Well, the rifle’s receiver is made from steel, so it is unforgiving. The rings and bases may be built from steel or aluminum. The scope is nearly always built with an aluminum tube. What does aluminum do under stress? It bends. When we have two independent scope rings that are not aligned, something has to give. A bent or stressed scope can cause all kinds of unforeseen problems, so we want to avoid that whenever possible. Ever remove a scope only to encounter ring marks on the anodized finish? That scope was not mounted in a stress-free manner.
The first step to solving this misalignment is to make sure we have a problem in the first place. Using a precision alignment tool accomplishes two goals. It tells you whether your rings need lapping and, if so, it places the rings in a parallel position so your lapping is as efficient as possible.
There are several tools on the market. Brownells, for example, offers 10 different products to align and lap scope rings. My choice is the Scope Base & Ring Alignment Tool sold by Echols Rifles. This is a precision-ground steel rod, made in a facility that produces things like nuclear missile components. It’s straight. These 1-inch or 30mm steel tubes are placed into the mounted rings to ensure alignment. Two sections of the tube have been milled away to allow the user to access the scope base screws while the bar is in place, which can be useful with horizontally split rings.
Lapping is not a foolproof solution to every scope mount alignment problem. If a scope is, say, .030 inch out of alignment, you could lap until your arms fell off and not solve the problem. In a perfect scenario, you are simply lapping the rings to smooth out the finish and remove any burrs that could mar your scope tube.
There is another factor at play here: By lapping the rings you are increasing the internal diameter. If you remove too much material from the rings, you’ll never achieve appropriate contact with the scope to hold it in place under recoil, no matter how straight the surfaces are.
If the lack of alignment is minor, say, a few thousands of an inch, it’s time to lap. Lapping kits are available from numerous sources, and most combine an appropriately sized rod with an abrasive lapping compound. The compound is applied to the inside of the ring halves and the rings are tightened just enough to contact the rod. The tool is then simply moved back and forth inside the rings until the desired amount of material has been removed.
You will likely be able to physically see what has been removed from the rings when the bar is removed, which is a good indicator of how much contact a scope and ring will have. To check your work, thoroughly remove the lapping compound from the rings and put your alignment tool back into place.
If the misalignment is too severe, consider a different brand or style of scope rings. If that doesn’t work, it could be that your receiver’s mounting holes are not aligned either vertically or horizontally. The solution here is to have a gunsmith drill and tap the holes out to a larger diameter (usually 8-40), ensuring that they run parallel.
Vertical alignment issues usually require shims. In an extreme case, custom mounts and rings can be machined to fit the idiosyncrasies of your individual rifle and action. For most individuals though, this solution would be cost-prohibitive.
Most rifle shooters and hunters slap the scope into the rings, tighten the screws and go shooting. This might be the easy way but it doesn’t mean that it’s the correct way. Take the time to use the appropriate tools to ensure that your rings are properly aligned and, if minor problems exist, give lapping a try.