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Modifying Musket & Carbine Locks for N-SSA Competition

By Dave France, 6th Wisconsin



This article is specifically tailored to aid in modifying musket locks, or the locks of carbines similar to musket locks, to improve the trigger pull. However, the principles are similar for locks that are quite different.

Several earlier versions of this article appeared in newsletters since 1976. Additions and improvements suggested by several N-SSA members who are accomplished gunsmiths have been added over the years.

Hopefully the article is helpful to everyone interested in working on locks. However, if you are not proficient at the type of work required through training or practice, you probably will need help from someone more skilled in gunsmithing to start. I personally have done most of the modifications suggested in this article, but I have learned (after ruining several parts) that the professional tool makers and machinists can do it much better. In addition to the skills professionals bring to gunsmithing, they use machine tools capable of more exact work than the files (and perhaps a Dremel tool) most shooters employ. Unfortunately all gunsmiths (particularly those who specialize in working on modern firearms) are not skilled in working on Civil War era firearms.

There are three ways we can improve the trigger pull to will help us in competition:

Reduce the weight of the trigger pull: A light trigger pull requires less force to squeeze the trigger; as a result, it is easier to keep the sights aligned on the target.

Eliminate creep or the mushy feeling detected with some locks: A lock with creep causes a change in feel as the trigger is pulled. The change in feel tends to make some shooters jerk the trigger or flinch. A firearm with a well-functioning lock allows the shooter to fire without detecting trigger movement.

Shorten the trigger pull: A shorter pull reduces the time required to squeeze the trigger and lessens the tendency to jerk the trigger or flinch.

Many skirmishers recognize a lighter trigger pull aids accurate shooting but don't recognize the benefit of eliminating creep and shortening the trigger pull. I have found both of these changes to be very helpful to accurate shooting.

Changes to improve musket and rifle locks predate the Civil War. The most common alteration was the addition of a wood screw. The screw contacted the sear and prevented it from engaging completely into the full cock notch of the tumbler The screw was placed in the wrist of the stock, running up alongside the trigger. This alteration reduced the force required to pull the trigger and the length of the pull. (We aren't allowed to make this change to our firearms, and I don't think it would be a very reliable means of improving the trigger pull.)

Figure 1: Illustration of Musket Lock Parts

Let us consider here the materials used in the original Civil War era Springfield muskets and rifles. Most of the metallic parts were iron. The iron used was wrought iron, a nearly pure iron with a very low carbon content. The wrought iron could not be hardened by heat treatment because of the lack of carbon. However, the iron parts could be case hardened (hardened to a depth of a few thousandths of an inch). Only hammers and a few minor parts were case hardened.

Wrought iron was extensively used in firearms because it could be readily forged, machined or filed, and welded. The most important advantages of wrought iron were probably its low cost and its suitability for welding. The only welding process available until many years after the Civil War was hammer welding. In hammer welding, the iron was raised to a high temperature, and the parts or edges were hammered together. (The barrel was formed from a flat sheet of iron by hammer welding the two edges together, and the bolster was welded to the barrel by hammer welding).

Wrought iron has very little use today. Modern manufacturing methods do not require the properties of wrought iron, and steel has replaced it. A reproduction firearm uses steel for the parts that were iron in the original.

Only a few parts in the Civil War era lock were steel. Steel was difficult and expensive to manufacture until the 1880s, but it had the advantage of being suited for heat treatment. The expense of steel limited its use to only those parts (springs, tumbler, swivel, and swivel pin) that required the increase in strength derived from heat treatment. Steel parts were through hardened (hardened all the way through the part) except the nipple that was locally hardened at the end of the nipple. Unfortunately the steel (crucible steel) used in lock parts did not have the alloying elements (chrome, nickel and several others) used in modern steel to improve strength. Consequently, steel parts in original locks occasionally fail. The full cock notches of some original tumblers shear off and mainsprings may break suddenly. You should have replacement lock parts available if you shoot an original firearm.

Prior to Making Modifications

Before starting to modify the lock, look it over carefully to see if there are any obviously defective parts; these must be replaced. Check the main spring; it should be as strong as possible. A strong main spring increases the trigger pull, but should never be thinned down to reduce the pull. A weakened mainspring increases lock time and may not consistently set off the percussion cap.

Many reproduction locks have a poor fit of the tumbler axle (or pin) to the holes in the lock plate and bridle, and a poor fit of the hole in the sear to the sear screw. The poor fit and other machining problems cause creep in the trigger pull. It is best to replace or modify sloppy fitting parts to eliminate this cause of creep. Obviously a good quality lock before modifications is a big advantage in preparing the lock. Some reproduction locks are of such low quality that they cannot be modified successfully without replacing all or most of the parts in the lock.

Examine the sear screw, sear, and bridle for proper function before modifying a lock. A well functioning lock must have a sear which moves freely with a sear screw that is tightened. If the sear screw binds the sear, and if tightening or loosening the sear screw affects the trigger pull, the trigger pull will not stay consistent. The sear screw should act only as a pin or shaft for the sear. The sear, bridle, and/or the screw may have to be changed.

Sometimes the only change required is a sear screw with a shorter thread length.

Completely read through the steps listed in this work and decide what will be done (there are some options) before starting any modifications.

Some problems with locks result from inadequate clearance in the lock mortise for the lock parts. It is important to check the inletting of the stock for the lock and trigger. If the wood binds against the trigger, or limits its travel, the trigger pull cannot be adjusted as desired. A light trigger pull or nonfunctioning half cock will result if improper inletting limits movement of the mainspring or the sear. If parts are replaced in the lock (primarily the mainspring or sear), or if moisture causes swelling of the stock, problems may arise due to interference with the wood. Many skirmishers epoxy bed their locks to reduce problems due to swelling.


Step 1. Modify the full cock notch to an angle that increases the trigger pull. You are probably wondering if you read that correctly or are wondering why make the trigger pull harder. The answer is simple. By changing the angle of the notch slightly, we will be able to shorten the trigger pull more than if the angle is left at the normal angle.

Figure 2.2 - Shape of the Metal to be Removed

The normal angle for the full cock notch is shown in Figures 2 and 2.1. Note the normal angle runs from the outer edge of the full cock notch to the center of the pin. Do not change the angle to a shallower one (Line A). Using the shallower angle will reduce the trigger pull, but a pull with creep will result. Line B illustrates the small angle (perhaps ten degrees or more) that increases the trigger pull. As noted earlier, this change permits the engagement of the sear into the full cock notch to be reduced more than would otherwise be possible, and consequently the length of pull can be reduced to a minimum.

Figure 2.2 illustrates the shape of the metal to be removed. If a thin grinding stone is used to machine away the metal, a small undercut shown in the illustration will result. The undercut is not harmful. This change to the full cock notch is very difficult to do well with a file. If filing is your only recourse, you are probably better off to skip this step.

Step 2. Modify the tumbler to reduce full engagement of the sear in the full cock notch. This modification will reduce the length of trigger pull and should be considered a must. It may be accomplished in several ways which are listed below. Remember, you will make one of the modifications described as Options 2A, 2B, 2C, or 2D but only one.

Option 2A) File or machine down the tumbler to the shape shown in Figure 3. Reduce the full cock notch depth to about the thickness of the end of the sear or about 1/32nd of an inch. Later, if the trigger pull is five pounds or more with an unmodified sear spring, the full cock notch can be further shortened without danger of reducing the trigger pull too much. The tumbler must be reshaped to prevent the sear from catching in the half cock notch as shown in Figure 3. After modifying the tumbler, the lock can be checked to determine if the sear will catch at the half cock by removing the springs from the lock and rotating the tumbler with the tip of the sear at a location just outside of the full cock notch. The half cock notch must not be lowered to the point that prevents the hammer from being pulled off the nipple; there must be a workable half cock.

This is my favorite means of modifying a tumbler if I do it myself. If the tumbler is not too hard, it can be done with tool makers files for the most part. It may be difficult to deepen the half cock notch with a file; in that case, a Dremel tool with a ceramic cut off wheel can be used. If the tumbler is very hard and difficult to file, another way of reducing the engagement of the sear is preferable.

Figure 3 - Mike Newhouse's tumbler for Springfield

One talented gunsmith, Mike Newhouse (1st Michigan Infantry), made a few tumblers for Springfield locks which were designed to the shape shown in Figure 3. They could be used in an original lock to improve the trigger pull without making other modifications. I bought several but eventually sold them to other skirmishers who wanted them very badly. Perhaps some enterprising sutler will have similar tumblers made and save some of us a lot of trouble.

At this point, check that the full cock notch and the end of sear are correctly shaped; incorrect shaping of these parts will cause creep in the trigger pull. The end of the sear should match up precisely with the full cock notch. See Figure 4. There must not be a radius at the bottom of the full cock notch (shown by the shaded area) that interferes with the tip of the sear. Regardless of what method is used to reduce sear engagement, the sear or tumbler must not be rounded (shown by the shaded areas). It may be necessary to change the shape of the sear tip to have the angle match up to the tumbler full cock notch.

Avoid heat treating steel parts of original firearms and reproduction firearms if possible. Heat treating of steel parts can only be done reliably if the alloy is known. Without knowledge of the type of steel, heat treating is not possible without some risk of making them too hard (and breakable) or too soft (and will wear). Both original and reproduction tumblers of many of the firearms we use today are through hardened. If a through hardened tumbler is modified, it should not be rehardened because of the risk of damaging a serviceable part.

Figure 4 - End of the Sear and the Full Cock Notch

However, some reproduction tumblers and revolver hammers have only a thin layer of hard metal over a softer base material. If the hard case metal is removed, the tumbler must be rehardened. If tumblers, springs, or other parts must be heat treated or case hardened, the reader is directed to the methods described in the article "Tuning Up the Replica Musket Lock" by Jim Leinicke in the July-August and September-October 1997 issues of the Skirmish Line.

Option 2B) Solder or use an adhesive to attach a thin piece of brass to the tumbler as shown in Figure 5. Many skirmishers use this method to shorten the length of trigger pull. The brass thickness chosen must be such that the notch depth is 1/32nd of an inch or less. The brass can be filed down thinner to increase the depth of sear engagement and the trigger pull if necessary. The common solders (mixtures of lead and tin) are not strong and fatigue easily. Eventually the solder may fail and the brass piece fall out because of a shock load which occurs at the solder joint when the hammer strikes the nipple. It happened to me at a skirmish, and it was a very unpleasant surprise.

Figure 5 - Brass shim on tumbler

Silver solder is much stronger and more reliable than the lead-tin solders. But the use of silver solder is a skill that has to be learned and it may be difficult to master.

An option to solder is an adhesive. My choice of adhesive is Loctite Black Max. I have used it for many years to retain a sight in a dovetail and to retain a sight blade in the base of a front sight. Black Max is more resistant to impact loads than super glue or epoxy adhesives and is much easier to use than soldering. If you don't think you can use silver solder, try Black Max.

Option 2C) Add a set screw or pin to the tumbler as shown in Figure 6. This seems to be the most popular method of reducing the length of trigger pull. However, this method cannot be used on some tumblers or revolver hammers. In the Sharps tumbler, for example, a pin or set screw would interfere with the swivel pin.

Figure 6 - Set screw or pin added to tumbler

Locate the set screw or pin carefully so it is normal (or perpendicular) to the sear where it touches the screw. If you decide to use a set screw, drill and tap for a #4-40 or larger screw. Increase the tap and thread size as large as possible; larger threads can be tapped with less risk of breaking the tap. A drill one size larger than normal can be used to reduce the chance of breaking the tap, but the threads in the hole will be very shallow. And shallow threads are not satisfactory for use with a screw that will be turned in and out to adjust the trigger pull.

Some skirmishers use the set screw to adjust the trigger pull. They turn the screw in to increase the trigger pull, or turn it out (reducing sear engagement) to reduce the trigger pull. This method of trigger pull adjustment does not permit the skirmisher to have the shortest possible trigger pull. It is better to minimize the length of pull and use another method of adjusting the trigger pull (to be described later).

Adjust the screw length by filing (on the end that goes in the bottom of the hole) to a length permitting the desired sear engagement (1/32nd of an inch or less). Make fine adjustments by filing the exposed end of the screw. Make sure to use a Loctite adhesive on the screw and turn the screw in until it bottoms; these steps will prevent loosening. If a pin is used, it can be soldered (or retained with an adhesive) in a drilled hole, and filed to the desired length.

Figure 7 - Weld bump

Option 2D) Heliarc weld a bump of steel on the tumbler to limit engagement of the sear in the notch as shown in Figure 7. Helicarc welding is electric arc welding with helium gas used to keep air away from the weld. Done correctly, it should produce a strong weld without raising the temperature of the tumbler and affecting its heat treatment. After welding the bump can be ground or filed down to give the desired amount of sear engagement.

Unfortunately, very expensive equipment is required for heliarc welding. You probably can only have it done by a welding shop or a machine shop with the correct equipment. If you can find a source for the work, the result should be a reliable, convenient way to shorten the trigger pull.

Step 3. Polish all the surfaces in the lock that move relative to each other while the trigger is being pulled. This helps to minimize any change in pull that would occur as the surfaces wear in, and it makes it difficult (or impossible) to tell that the trigger is actually moving. Polishing stones (Arkansas Stones) with a triangular cross section (made for machinists, tool makers, etc.) are handy for smoothing the full cock notch of the tumbler. A more easily found tool is the "Diamond Hone & Stone." Take great care to not round off edges on the tumbler and sear that should be square. (Refer to Option 2A).

At the same time, polish all the surfaces that move relative to one another while the hammer is moving - this will help to reduce lock time slightly. Fine emery paper or cloth, backed up by a hard surface, can be used to polish the inside surface of the lock and bridle.

Final Adjustments

After the work described above, install the lock in the firearm and check that the sear does not catch in the half cock notch during firing. Also check the trigger pull with a spring gage or weights and recheck frequently while making final adjustments. Determine by testing if the trigger pull is changed by installing and tightening the lock plate screws. If the lock plate screws change the trigger pull, install them each time the trigger pull is checked.

At this point, the trigger pull should be in the five to ten pound range. If less than five pounds, it may drop below the desired 3 1/2 to 4 pounds as the parts become more polished with wear. If more than ten pounds, it may not be possible to lower the trigger pull to the desired level by modifying the sear spring. If the pull is not within the desired range, check the lock over carefully for problems. The most common problems are interference of the trigger or lock parts with the wood, an incorrect full cock notch angle on the tumbler, too much engagement of the sear in the full cock notch, and binding of the sear.

If the full cock notch angle must be changed, refer again to Step 1 and Figure 2, and do not change the angle below the normal angle of Figure 2.1.

Let's consider here exactly what affects the trigger pull. As the shooter pulls the trigger, the force applied is resisted by friction and the sear spring (it is compressed as the sear moves). There is friction between the trigger and sear, between the sear and bridle, and at the sear to tumbler interface. The highest friction force occurs between the tip of the sear and the full cock notch because of the high load (created by the main spring) which forces these two parts together. As noted earlier, the full cock notch angle and the amount of engagement of the sear in the notch can be used to change friction at this point. Polishing of parts also reduces friction.

We can reduce friction farther by adding grease to the lock. Grease applied at the full cock notch has the greatest effect upon trigger pull. Using the more common kinds of grease will only slightly change the pull if the parts have been polished well. The change will probably be about 1/2 to 1 pounds.

There are two advantages for using grease in the lock:

1) Grease will reduce the large variation in trigger pull that may occur because of friction changes caused by humidity. On a rainy day, or even on a very damp day, the trigger pull will drop because of the lubricating effect of the moisture in the lock. With the use of grease, the change is greatly reduced.

2) Grease will reduce the trigger pull. But the disadvantage of using grease is that it must be reapplied to maintain a consistent trigger pull. If grease is used in the lock, it will reduce the trigger pull slightly, but after a day of shooting the trigger pull will rise again.

I always use grease in revolvers to help soften the deposits that collect there. If used as part of the process of adjusting the trigger pull, the grease should be applied to the full cock notch and on all the surfaces of the lock parts that move relative to another part. I have used Shooter's Choice and Gunslick, greases made specifically for firearms, but have found that white lithium grease works just as well and is very inexpensive.

Do not use powdered graphite, molybdenum disulfide, or other super lubricants in a lock. I have tried them, and they reduce the trigger pull much more than ordinarily desired.

For musket locks of the type shown in Figure 1, make final reductions in the trigger pull by modifying the sear spring.

Figure 8 - Reduce stiffness in sear spring

Reduce the stiffness of the sear spring (to reduce the trigger pull) as illustrated in Figure 8. The spring can be reshaped by filing or grinding. If it is ground, dip the spring in water frequently to avoid overheating and softening. Reshaping, as shown in Figure 8, will not increase the maximum stress in the spring, and it is unlikely to break. (I have never had one break.)

Take care not to weaken the spring excessively. If too weak, the spring will not force the sear into the tumbler full cock notch, and the lock will not function correctly. If the sear spring cannot be reshaped to reduce the trigger pull to the desired level, go back to Step 1 and/or Step 2.

If the sear spring is used to adjust the trigger pull, prepare and have available extra springs to raise the pull. I usually have two or three extra sear springs for each musket lock that are weakened to various degrees to provide trigger pulls higher than the sear spring in the lock. As noted earlier, moisture and wear can cause the trigger pull to drop. It is best to check the trigger pull every day of competition, and change the sear spring if necessary to raise it.

In some firearms, there is no sear spring since there is no sear. In revolvers and Smith carbines, the trigger engages the tumbler or hammer, and the trigger spring functions as the sear spring in the musket lock. In the Sharps lock (and other back action locks) the sear spring is one end of the main spring.

Do not use the trigger spring to adjust the trigger pull of revolvers. In revolvers, the trigger spring has little effect on the trigger pull and adjustments to the pull should be made by the methods described in Step 1 and Step 2.

For the Sharps carbine or rifle, use the mainspring retaining screw as a cam to adjust the trigger pull rather than use the sear spring end of the main spring:

• File off the projection of the mainspring that rests on the screw.

• Then, file the screw head to a cam shape.

• Turn the screw to raise and lower the spring, and measure the effect on the trigger pull.

• Weaken the sear spring end of the mainspring to adjust the trigger pull if necessary, and/or change the angle of the full cock notch or sear engagement if necessary.

• Take care not to install the spring in the lock without the screw being in place. The sear end of the spring can be overstressed and damaged without the screw.

As a final step in preparing the lock, add a thread adhesive to the screws in the lock. The most common brand of adhesive is Loctite. Loctite Threadlocker 242 is a medium strength adhesive intended for use with small screws. Do not use stronger adhesives that may prevent disassembly of the screws without heating with a torch. An adhesive should be applied to all the lock screws, but it is more important for the hammer screw and the sear screw (they are most prone to loosen). Excess adhesive (and gumming up the lock) may be avoided by applying it to the tapped holes in the lock plate from the outside of the plate.

Follow the following procedure:

• Clean the screws and tapped holes with alcohol.

• Start each screw in the tapped hole after the lock parts are assembled.

• Add the Loctite to each tapped hole from the outside of the lock. (Of course this step does not apply to the hammer.)

• Work each screw back and forth a few times.

• Finally wipe off the excess Loctite on the outside of the plate.

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