The Basics of Double-Gun Lockup

In several Middle-European countries, the favorite hunting gun is a "drilling." A drilling has two shotgun barrels mounted side by side with a rifle barrel fixed directly beneath them. All three barrels share the same receiver and stock. These exotics probably don’t enter your shop very often, but the information that follows can serve you well when you encounter a double shotgun that has "gone loose." Case in point: A good client recently brought a drilling into my shop for major work.

I have known the gun’s owner, John, for over 30 years. This was originally his father’s gun, and John has been using it annually for over 40 years. For most of those years, I’ve checked this gun out at the end of the season, and we’ve discussed upgrading it for the last five or six years. This year, it’s getting a full rebuild, and I thought I would share some of the procedures with you. The Basics There seems to be a common belief among double-gun enthusiasts that if a gun is loose, it has a worn hook.

The common remedy is to make anew hinge pin or add metal to the barrel hook, and then fit the breech back onto face. Doing this will tighten up the gun, of course, but this oversimplified approach doesn’t take into account the other locking, bolting, and tightness aspects of the gun. In many cases.

It’s just "half a fix." There are several elements that must be in proper relationship: 1) The hook and hinge pin must mate and hold the breechface as tight as possible against the standing breech. 2) The locking bolt must engage and mate with the barrels’ underlugs and hold the barrels down securely, and yet still have a remaining bit of forward movement to compensate for wear. 3) The forend iron plays a significant role by holding everything together.

The gunsmith must understand that the barrel hook, hinge pin, and ironall threeform a joint on which the barrels hinge and rotate. All three of these elements must be in proper relationship for the gun to have along life of correct function. If you repair just one element, the life of the repair may be rather short. The Job at Hand The work order for this drilling went like this: Make and fit anew hinge pin, including smoking the breech back onto face. Half-sole the locking bolt and refit it.

Make anew (Greenerstyle) cross bolt. Weld and refit the recess slot on the forend iron for a tighter fit. When rebuilt and tested, it would then be stocked and metal finished. The work began with the use of stones to clean and smooth the window in the receiver in which the locking bolt moves. It is the bottom of the locking bolt that engages and holds down the barrels. Inspection determined that the locking surfaces need to be moved down a full .003 inch.

Milling a recess on the top surface of the locking bolt allows a place on which to attach (with soft solder) a thin piece of 01 tool steel (see Figure 1 on the next page). Once this spliced piece is attached, the openings for the lugs can be milled and stoned back to the remaining original lines. Surface grinding is used to bring the part correctly to a point close to the needed .003 inch.

Refitting the part to the attached receiver and barrel assembly is done using hand stones on the spliced new steel, leaving the original locking bevels on the bolt alone. The point is to retain A section of the locking bolt is visible in the photo here.

Figure 1. the original locking surfaces, yet move them down. At this point, I bring the fitting to about 98-percent completion. I make all fitting adjustments to the altered locking bolt, not to the barrel-lug recesses. We want smooth and clean locking and unlocking, but don’t bring it to completion just yet. Experience is of some importance here regarding the value of the repaired part. Hardness is something to consider.

Before you work on the part, check its hardness, either with the old gunmaker’s "file test" or with a Rockwell tester. Locking bolts are generally not particularly hard; files usually cut them without much effort. A Rockwell-C in the low 30s should do just fine. If this hardness cannot be maintained, the face area, will hold fine and should not affect the original hardness of the part as long as you keep the temps low. A half-sole repair is an acceptable way to repair a locking bolt.

The other alternative is to mill out anew part. The raw 01 steel has sufficient toughness to function well for another generation. This particular drilling had a hinge pin that was relatively easy to remove. Fully assembled, the breech would close and lock on a .003-inch feeler gauge at all locations around the breech area. A thicker gauge would go in at different locations and, at some spots, a .004-inch gauge would not go in.

An additional .002 would be needed to allow enough material to be removed from the breech for a surface as flat and level as possible. So 0005 inch less than the diameter of anew hinge pin which will be pressed in. This ensures the barrel lug will be cut to the same dimension and will mate well with the pin. correct thing to do is to make anew locking bolt from known steel and follow the guidelines for hardening and drawing.

Soft soldering the splice on, as long as there is sufficient surthe target amount to move the barrels back is .005 inch. We can’t always have everything we want. I am sure you are aware that the standing breech on double guns is not 90 degrees. Actually, 88-1/2 degrees is about average for most guns. This is to allow the needed room for the barrels’ bottom edge to clear, as it swings open in an arc.

Because of this, and the fact that the wear on the breech is a bit uneven and the standing breech may not be perfectly flat, it’s unrealistic to expect to get a full 100-percent even contact on the barrel breech. One-hundred percent is possible on the bottom half of the breech, but 98-percent full, even contact on the top half is doing well. Moving the barrels back a full .005 inch will give usa great fit. That, in conjunction with the other work, will give this gun another full generation of use.

To get the .005 inch we need, the new hinge pin must be made .010inch oversize. Drill rod works just fine for this, as long as its hardness is comparable to the hardness of the barrel-lug hook. This does need to be precision work, though. Working to within .0005 inch should be strived for here. As I said before, we can’t

have everything we want, so .010 inch over is our target size, but we must consider what hand reamer sizes are available that are close to what we need, and that will determine what we’ll use for actual dimensions.

The procedure here is simple: 1) Make the new hinge pin to exactly the size you need. 2) Use a series of hand reamers to open the hinge-pin hole to exactly .0005 inch less than that, and then press it in. 3) Smoke (fit) the 010 inch larger in diameter than the old pin. breech back onto face, using a smooth cut file and stones. The locking bolt and forend should both be in place during this operation. In the photo, I made a mistake; I cleaned the oil off for the photo and forgot to reattach the iron.

That’s the only mistake I’ve made this year. Have the iron on for the reaming. This job was fitted to 98percent completion at this time. There remains three more things to do on this job. The forend iron plays a big role in holding it all together, and a tight fit against its lug is important. Since the gun will be casehardened after stocking, this recess is TIG welded and dressed. You remember that we took each of the jobs to 98-percent completion?

Well, now with each part of the job at 98 percent, we bring the whole job to 100-percent completion. Why put the gun on face when you have a loose locking bolt? The final job should open and close smoothly, and when locked up, it should be tight. The top lever should be just right of center. It should work like it’s supposed to; you shouldn’t have to tell the client: "That will wear in." The final part to complete is the Greener cross bolt.

The gun is assembled and locked, the hole is reamed, and then anew bolt is turned, fitted, and engraved. Since it will be casehardened, low-carbon rod is used. The parts of a double gun or drilling are kind of like the cars of a freight train. They each pull or interplay with each other, and you can’t change the value on one part without changing the value of another part.

Conclusions In the final analysis you might ask: "Why go to all this trouble?" John’s father used this gun and passed it down to him, and now John wants it rebuilt and refinished so he can pass it down to his grandson’in as good a condition as it was in when he received it. In other words, he wants it to be ready for another generation of use. The gun is already about 80 years old, and well used, but well taken care of’countless heads of game and birds have fallen to it.

If I do my job well, it should be good for another 80 years before needing another rebuild, which would certainly be possible. Try doing that with the throwaway guns they’re making today! And that’s why he going to all this trouble. I Reprinted with permission from the author and the American Custom Gunmakers Guild, Inc.

AGARAD6 ARMORER’S COURSE SPREAD, AD AMG VER. (pick up from 12/06 issue)

AGARAD6 ARMORER’S COURSE SPREAD, AD AMG VER. (pick up from 12/06 issue)