February 20, 2013

Building Custom Shotgun Mounts

When there isn’t a scope mount made to fit the shotgun you’re working on, don’t worry about it. Just use a blank to make your own.

Both B-Square and Aimtech happen to make a nice line of scope mounts for shotguns that are advertised as requiring no gunsmithing. For the most part, that’s true. These mounts slip over the receiver like a saddle on a horse. Removing the trigger mechanism retaining pins and replacing them with the slightly longer ones supplied with the mount secures it to the receiver.

Up to that point, no gunsmithing is needed. Then it comes time to bore sight the scope. Most do-it-yourselfers usually don’t have at their disposal an optical collimator or other bore sighting device, any one of which saves considerable ammunition during subsequent sighting-in sessions.

Several companies offer optical collimators, which are among the most precise of the bore sighting instruments available. They are not outrageously expensive and can be used with any shotgun, rifle, pistol or revolver. For the serious practitioner, they are a good investment that will pay for themselves many times over.

American Gunsmith, Book of the Shotgun. Chapter 21

Courtesy, American Gunsmith Magazine

Mounted on an Ithaca Model 51 Featherweight, the finished base had its forward portion set slightly back from the interface between the barrel and receiver to assure lock-up.

Disregarding the profits, you should know that “no gunsmithing” doesn’t always mean what the description implies.

There are at least two problems with no gunsmithing mounts to which the only solution is considerable gunsmithing.

They aren’t made for every shotgun, especially older ones. And some owners just don’t like them for reasons of their own.

In either of those situations, the only way you can satisfy a request for a scope mount is to manufacture one. A common starting point for most of us is B-Square’s Blank Weaver Bases. Made of black-anodized aluminum, these range from 0.147 inch to 0.650 inch in height and are 10 inches long.

If the shotgun you’re working with is a Remington or Winchester, the available bases are pre-contoured to fit them. There are three universal bases offered to accommodate other brands but you’ll very likely have to re-contour their bottoms to achieve a good fit between the mount and the receiver.

The example used here is an Ithaca Model 51 Featherweight, for which an 0.396-inch high universal base was chosen after determining it was the closest match available for the shooter’s center line of sight through a mounted scope. Actually, it was “closest” by 0.50 inch, which meant it had to be lowered to 0.346 inch. Our first step, however, was to use a hacksaw to cut the base stock into two pieces, one 4 inches, the other 6 inches. The longer piece would eventually become the scope base. There’s nothing fixed about those lengths. It purely depends on the point at which the receiver becomes thicker aft of the ejection port. The forward top section of most shotgun receivers is only thick enough to accommodate two, at the most three, screw threads. (Some, like the Franchi, are simply too thin for tapping.) Because of this lack of metal, and the resulting limited thread contact, the forward portion of the base will require drilling and tapping for two 8-40 mounting screws. At the rear, where the receiver is thicker and the purchase more secure, you can get away with one 8-40.

After locating the holes in the base, drill and tap them, then use the base as a guide to locate the holes in the top of the receiver. The perfect tool for this is a drift punch, with a diameter slightly less than that of a number 8 screw, on which the end has been ground down to a sharp point. You can also purchase a set of #6,#8 and #10 center hole punches and save your drift punches for drifting.

Naturally, the barrel, bolt assembly and stock must be removed before you do any machine work on the receiver. Once they have been, you can drill and tap clear through its top without having to worry about damaging any internal parts.

At this point, you have to check the fit between the base and the top of the receiver. If you can see daylight between the two, the bottom channel of the base will need to be machined out with an end mill until it sits flush. An end mill that matches the width of the receiver’s top flat makes this step pretty simple. On receivers that have no top flat, the bottom channel of the base will have to contoured to match with a ball end mill.

Since these bases are aluminum, the machining is easily accomplished and so is any lowering that might be necessary. Simply place the base on a belt sander, hold it down firmly and let the sander do all the work. You’d better wear some thick gloves or use a hold down stick; the things get really hot when their bottoms are being scraped away, but whose wouldn’t?

One end of the base stock you cut will still be bright aluminum, so touch that up with aluminum black before mounting the base on the receiver. Tightly screw it down, being careful not to strip out any threads in the process. Turn the receiver upside down to see if the screws protrude on the inside. If they do, mark and remove them, then cut them off according to your measurements. Re-thread the screws with an 8-40 die, mount the base and check again for any intrusion.

The mounting screws must be absolutely flush with, or very slightly below, the receiver’s interior. If they aren’t, neither the barrel nor the bolt can be reinstalled. The gun has to go back together as easily as it came apart. If you force it despite of an invading screw, you’re inviting problems—a rock-solid jammed bolt comes to mind—you don’t need.

To permanently mount the base, run a thin coat of AcraGlas along its bottom channel and use any kind of good, thread locking goop you have on hand to secure the mounting screws.

If you’ve taken your time all along the way, you’ll end up with a mount that not only looks good, but will probably serve its intended purpose for as long as your owner owns his gun. ★

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