Morse Taper and Shotgun Repair

A machine taper secures machine tool spindles, with the taper fitting inside alike-angled socket. The old mechanical Morse principle, a selfholding variety of taper, helps in understanding failures of the shotgun bolting systems and their repair. The evolution of muzzleloading firearms into breech loaders created anew challenge for the gun makers: the design of a strong Breech Bolting System (BBS). This mechanical portion of the gun ensures a tight breech seal for the duration of the shot.

The bolting system takes a hard beating on every shot since the pressure in the chamber is trying to open the breech. This is universally true for any firearm and any bolting system but the subject of this article is a BBS of the break-open shotgun. Right after the first appearance of the break-open design of Casimir Le Faucheux, patented in 1836, his hinge-and-bite bolting of the pin-fire shotgun brought up disbelieve and skepticism on the part of traditionalists.

This is a natural Skeptics pointed out weaknesses in the BBS. The bolt is controlled by the under-lever. reaction since muzzleloading firearms were at their peak of development and popularity then. The major point of skepticism was the weakness of the hinge-and-bite BBS, holding barrels from opening during the shot. Indeed, this simple design was not strong enough. The opening force of each shot was quickly loosening this bolt, making it inefficient. Improvement of this BBS was a real challenge.

After the presentation of Le Faucheux’s gun at the Great Exhibition of 1851 at Hyde Park, skeptical British gunmakers took this challenge on and created the whole specter of the shotgun bolting systems, competing for the best one. Everything has been tried: single or multiple bites, third fasteners, Greener cross bolts, "doll’s head" extensions, Kersten locks, trunnions and hinges, as well as bottom, central, and This schematic shows the bolt engaged with bite and the angle of engagement. top bolts.

Each one has advantages and disadvantages. The marathon of these designs started out at the break of 19th and 20th century and never stopped. The major goal is to create a BBS which is strong enough to keep closed on every shot, to withstand deformation ("keep face") for as long as possible, and provide convenient manual opening and closing. Now, after more than 100 years of this search, the dominating BBS design of the break-open shotgun can be generally illustrated as shown in the sketch in this article.

The sketch depicts one bolt and one bite, which is typical for the Browning-type cluster of shotguns. However, a wide variety of popular designs like the top bolts in the Remington 32 and Krieghoff, locking pins with Beretta, the rotating bolt found in Fox, and many others can be merged into this group for the failures of BBS discussed in this article. These failures are self-opening and self-locking of the breech at the shot. Excluded from this group are the true Greener crossbolts and their variations.

In other words, the cross-bolted ac-

tions never open and never lock themselves at the shot. Morse Taper Surprisingly, principles of BBS are tightly related to the principles of a Morse taper, invented in 1864 by Stephen A. Morse for reliable joining of two machine components. We all are using lathes and mills where a Morse taper is a usual accessory. The major quality of this taper is its self-locking ability. There is a number of different tapers in machinery. Some are selflocking as Morse or Jacobs tapers while others are self-releasing.

To disconnect female and male parts of the self-locking taper we Fr is a force trying to open the breech. Ful is pushing the bolt out of bite, an unlocking force. Flk is a force exerted on the bolt by the top lever spring. Fn does not contribute into any bolt motion. Qis a bolt and bite angle of engagement. The angle of the locking bolt is greater then 7° and therefore is self-releasing. need to apply force. With self-releasing taper it’s the opposite and we have to apply force to keep them engaged.

Everything depends on the angle of tapering. For Morse it is around 3°. The smaller the angle, the stronger the self-locking; the greater is the angle, the stronger the self-releasing. The borderline is at 7°. Every taper of 7° or less is self-locking and greater than 7° is self-releasing. As the dominating BBS design illustration shows, the bolt-bite engagement of the gun is a joint controlled by the taper angle. If so, the bolt-bite taper should not be less than 7°.

It also should not be much more than 7°, otherwise the locking spring needs to be much stronger, which prevents easy opening of the gun. This way, we come to a principle: the bolt-bite taper should be as close to 7° as possible. Let’s look at selfopening of a BBS. It happens quite often. In most cases it is a result of a natural wear and shot-forging of the bolts and hinge. It is not noticeable right away.

The first indication of selfopening is a position of the top lever turned to the right from its original position after the shot. This will be shown by the position of the top lever. The left side labeled "A" shows the position of the top lever before the shot and "B" is its position after the shot. If not fixed, this problem grows, ending up with barrels breaking open. Self-opening often comes with offface condition when the barrels become loose in the action.

I have never seen any danger associated with this unpleasant situation but this is a failure which needs to be repaired. Otherwise, besides opening, the ejectors and firing pins stop doing their job. Also, the point of impact on the second shot changes its position. Consider the forces applied to the bolt during the shot. There is a distribution of forces applied to bolt during the shot. Here, Fr is a force that is trying to open the breech. In other words, rotate the barrels around the hinge.

I call it rotating force. Ful is pushing the bolt out of bite and is called unlocking force. Flk is a force exerted on bolt by the top lever spring. It pushes the bolt inside the bite and is called locking force. Fn does not contribute into any bolt motion and is neutral. Qis a bolt and bite angle of engagement.

The area is blued to better show the brazing compound where a steel plate has been added and angle adjusted to about 7°. The metal template shows the 7° angle. For self-opening BBS it is always greater than 7° and making the taper self-releasing. Note that selfopening can happen in only one way, when the bolt retracts back from the bite into standing breech and leaves the barrels unlocked. The resulting force Fres affecting the bolt can be described as Fres = Ful – Flk = Fr * sin(Q) – Flk.

If Fres < 0, the gun is locked. If Fres > 0, the bolt is retracted, the gun is unlocked, and the barrels open. The formulae shows that for the greater angle Qand a weaker top lever spring, the self-opening is more probable. The repair of self-opening involves installation of the stronger spring and/or changing the angle Qto a smaller one. The later is more efficient, especially when the top lever spring is of the complicated V-shape and hard to replace.

Important is that by reducing the angle Qalone we can achieve as little unlocking force as desired and make sure that barrels stay locked. We just need to remember that making this angle smaller than 7° will convert it to a self-locking, which makes opening of the barrels really hard. Sometimes turning the top lever for opening the barrels after the shot becomes impossible, as discussed below.

Consider a modified bite angle, shown here with a blued area to better show the brass brazing compound where a steel plate was added and angle adjusted to about 7°. The angle on the bolt itself has to be modified accordingly. This is shown as take two with the bolt after adding metal by welding and adjusting the angle to 7°. The metal template of the 7° angle is quite useful for adjusting bolt-bite. The self-locking BBS is somewhat puzzling.

Imagine that the gun was used for awhile without any problems and suddenly you cannot turn the top lever for opening the barrels. What is going on? When I first faced this problem my guess was that the friction in the bolt-bite somehow grew up. After polishing the surfaces of the bolt and bite, the problem with opening became even worse. This seems strange but is easy 4°, which is self-locking taper. Bite angle A and bolt angle Bof the self-locking shotgun are shown to be less than 7°.

Bolt Bis horizontally positioned in the vise before measuring angle. to u ndersta nd knowing that it is a Morse taper. These self-locking tapers "glue up" much stronger when their surfaces are closer to each other. So, in the process of working together the bolt and bite polish themselves to a closer match and bond stronger. Deep in physics the Morse principle is explained by inter-molecular forces but this is beyond our scope. A self-locking BBS does not open, even with a force of 10Kg (about 22 pounds).

In fact, this gun opens at about 26 pounds of force, making it not operable. The bolt-bite angle of this gun taking 26 pounds of force to open is 4.4°, which is self-locking taper. It needs to be increased to 7°. To increase the angle one has to first add metal on the bolt and bite and then adjust the angle. I add metal on the bolt by welding and by brass brazing the steel plate.

Conclusion Two widespread failures of the Breech Bolting System of the breakopen shotguns can be explained by the principles of the Morse taper. These failures are self-opening and self-locking of the barrels. First is when the gun is opening at the shot. The other is when the opening after the shot becomes hard or even impossible as the top lever cannot be of their repair is to adjust the bolt-bite engagement angle to about 7°.

All said above is associated with shotguns but is true for any break-open firearm, including rifles, drillings, single shot, or combination guns with breech bolting shown here. AG