Whenever someone asks Doug a Thompson question that generates an answer that would probably be of interest to others, the question (perhaps edited) and the answer will be posted here.

These are the questions and answers that are covered in this section:

1921 Buffer Discs

1928 to1921

Mud Grooved M1 Bolts

M1 v M1A1

Receiver Steel Alloys

Grip Mounts

Firing Pin Springs

Shooting A Colt

Trip Projection

Lefty M1/M1A1 Bolt Handle

Bent Actuators

TSMG Barrel on a N/K Gun

Interchangable Thompson Rear Sights

Making a Machine Gun

Gun Info

Receiver Steel

1921 Serial #1

Interchanging Barrels

Thread Sizes

Sling with Vertical Foregrip

1921 v 1928 Bolts

Gripmount Styles

M1 Improvements ??

Kinked 1928 Recoil Spring

Barrel Removal Specifications

Worn Barrels

Ring Front Sights on Threaded Barrels

Parts Kit Semi-Auto

Aluminum TSMG

1921/8 Safety Lever Handle

M1A1 Bolt Handle Grooves

1928 Bolt Ramp

M1A1 v M1 TSMG Reliability

Interchanging Trigger Housings

Models of 1922 & 1923

Drum on M1

Savage & A-O Trigger Housings

Receiver/Trigger Housing End Mismatch

A5 Barrel Threads

Colt Sling Swivel

WW2 TSMG Finish

M1 & M1A1 Trigger Housing Serial Numbering

Vertical Foregrip Bolt Sling Swivel

1928 & M1/A1 Recoil Springs

TSMG Won't Fire in Semi-Auto

Rear Grip Lock Washer

Types of Sight Rivets

Bolt Thread Size



                                                       Bolt Thread Size

QUESTION : I understand that all the wood holding bolts on the Thompson are threaded #14-24. Why didn’t they use something common like 1/4"-28?

ANSWER: “Common” is subject to change. In 1920, #14 was in common usage under the old National Thread System. That would be the answer to your question if the shanks of the bolts were #14 (.242") in diameter because , generally, the bolt shank and thread are made the same diameter to reduce the cost of manufacture. But, the shank diameter on the bolts is 1/4" (.250"). So now you have me. I don’t know.

It makes no sense to me that the bolt shank and thread diameter are different. If I were redesigning the Thompson today, I would keep the shank at 1/4" diameter and make the thread size 1/4" diameter to match. I don’t see any downside of increasing the thread diameter .008". As a matter of fact, the advantage would be two-fold: less cost to manufacture and better gripping of the threads due to the increase in diameter which translates into increased length of engagement. I would also choose 28 threads per inch (tpi). We could use 24 tpi like the original but, again, the extra threads provide for more engagement and 24 tpi is somewhat of a special today. This would be particularly advantageous on the forearm/foregrip because the grip mount has less than 1/4" of threads.


                                                    Types of Sight Rivets

QUESTION : Thompson guns are fitted with adjustable rear sights and 2 styles of fixed sights. Do they all use the same rivet to fasten them to the receiver or are different types of rivets required?

ANSWER: All Thompson rear sights use the same rivet to secure the sight to the receiver. All three standard Thompson rear sight styles have the same rivet pattern, material thickness and hole dimensions.

                                                        1921 Buffer Discs

QUESTION : How many fiber discs are supposed to be inside the 1921 buffer?

ANSWER: There is no set number. The discs vary but are about 1/8" thick. With the pilot inserted in the buffer, the gap between the end of the buffer and the shoulder on the pilot is supposed to be .150" to .180" (that’s about 5/32" to 3/16") with the discs fully compressed. Of course, there really isn’t much buffering capability provided by a compressed stack of hard fiber discs, but that was the best material available in 1920. The only thing that is really important is that there is a gap so as to prevent the pilot from hitting the back of the buffer. If desired, a disc can be sanded thinner to get the exact dimension.

                                                            1928 To 1921

QUESTION : What is the difference between a 1921 Thompson and a 1928 and can one be made into the other?

ANSWER: A 1921 receiver is identical to a 1928. Except for 4 parts, the two models are identical. Those 4 parts are the actuator, recoil spring, buffer and pilot. Change those 4 parts and one model becomes the other. Of course, I am speaking about dimensional differences, not manufacturer and finish variations. The 1921 actuator is much lighter than the 1928 and accommodates a much larger diameter recoil spring. The 1921 buffer is a steel top hat-shaped housing that is filled with fiber discs whereas the 1928 buffer is a fiber washer. The 1921 pilot is smaller than the 1928 but both have the same protrusion out the back of the receiver. The 1921 recoil spring fits over the buffer whereas the 1928 recoil spring fits over the pilot. The 1921 pilot fits into the buffer, acting like a piston against the fiber discs.

                                                Mud Grooved M1 Bolts

QUESTION : I have heard that some M1 bolts had mud grooves cut in them. What do you know about that?

ANSWER: All I can tell you is that I have seen M1 bolts that have a mud groove cut into the full length of the top of the body of the bolt as well as on the left side. I have a few of those bolts. I have never seen an M1A1 bolt with the grooves. However, none of the drawings or other documentation I have mentions the grooves. I am also unaware of any contemporary writings that discuss the grooves.

Regardless, it does seem like it might have been a good idea since the M1 gun has no open spaces inside the receiver like the 1928 has. The grooves may not have been attractive to the Army, though, because the bolt would be lighter than the non-grooved bolt and so the rate of fire would be higher.

                                                                        M1 v M1A1

QUESTION : What is the difference between models M1 and M1A1 TSMGs and which is better?

ANSWER:   An M1 and M1A1 are virtually identical with the only real difference being the bolt assemblies. The M1 has a bolt that incorporates a firing pin, firing pin spring, hammer and hammer pin mechanism similar to the 1921 and 1928 guns. The M1A1 bolt deleted all those parts and simply has the firing pin machined onto the bolt face. The shank at the front end of the M1A1 bolt is also slightly longer than that on the M1. The two bolt assemblies are interchangeable. Substituting one for the other changes the gun model.

The M1 bolt cannot detonate a cartridge until it closes completely against the front end of the bolt cavity which is what drives the hammer. The M1A1 bolt with its longer shank is stopped by the cartridge. Whenever a cartridge stops moving forward, the M1A1 bolt detonates it.

The two models operate identically for the most part. Because the M1A1 bolt is stopped by the cartridge, a slightly longer cartridge or a little dirt in the chamber or a primer set slightly too deep in the shell case, will still be detonated by the M1A1 bolt whereas the M1 bolt may not detonate the cartridge.

Which is better depends on your point of view. The fewer parts in the M1A1 bolt means fewer parts to break which adds to reliability, fewer parts needed to be stocked and the gun fires more reliably than the M1. That appeals to the military. But, the downside is less safety. Since the M1A1 bolt will detonate a cartridge when ever the cartridge stops moving forward, an obstruction in the chamber can result in a dangerous blast of brass fragments and gas blowing out the ejection port and a noise that will deafen the shooter and everyone else close by.

In my opinion, the M1A1 is the only way to go for the military which doesn’t have safety as its primary concern. But for the civilian shooter where safety is the primary concern, the M1 is the better choice.


                                            Receiver Steel Alloys

QUESTION : The various Thompson receiver drawings specify N2, 2330, 1340 and 1141 as the steel alloy used. Why was it changed and what should be used today?

ANSWER : Colts all used N2. By the time of WW2, N2 was obsolete. 2330 & 1340 alloys were used during WW2 but the final alloy used was 1141. The science of metallurgy was rapidly developing at the time. The goal of any designer is to choose an alloy that provides the characteristics required by the product, machines the most easily and makes the product at the lowest overall cost. 1141 machines nicely, is heat treatable since it is a medium carbon steel, was readily available at low cost and was, therefore, a good replacement for all prior alloys used.

1141 is available today. Yet, some people are using 1018 to make receivers. 1018 is "hardware store" steel not meant for extensive machining. Its appeal is that it is readily available, cheap and comes in a variety of sizes, but that is because it is meant to be used as is with little subsequent machining. 1018 is not a good choice for Thompson receivers. The other alloy some people use today is 4140. That's ok but not a good choice because it is more expensive, more difficult to work with and offers no advantage over 1141 for a Thompson receiver. If pre-hardened 4140 is used, it is difficult to machine and so people cut corners and don't produce a really detailed receiver with a nice finish. If annealed 4140 is used, it must be heat treated after machining or it will not meet Thompson specifications. People use it because they know about it, it's readily available and is used extensively for high power rifle receivers and most all commercial grade barrels and because people did not know what steel alloy was actually used until I started providing the original drawings.

A major factor in making a part that requires extensive machining like a Thompson receiver is the tendency for the steel to twist as the inherent stresses created in the steel when it is produced become unbalanced as material is removed. The best way to avoid that is to stress relieve the material prior to machining. Heat treatment also creates stresses which cause the part to twist. Proper selection of the alloy used can reduce these problems. However, the best way to produce a perfect part is to choose the best material for part and then harden and stress relieve it before machining. This works well with the Thompson because the Thompson receiver is not really hard like a file. It is hardened only to the point of providing the required strength. That is why it can be cut with a hacksaw but if welded, it becomes very hard.

Therefore, selection of an alloy to use is based on a number of factors. I make all my receivers from the same alloy (1141) used on the last receivers made during WW2. It worked just fine for hundreds of thousands of Thompsons so why change?

Although, the original receivers were heat treated after machining, I use pre-hardened and stress relieved material. That is why my receivers do not twist if someone finishes one into a working gun (with proper license, of course).

                                                  Grip Mounts

QUESTION: What are the various widths and styles of grip mounts?

ANSWER: All Thompson grip mounts are ˝" wide and are interchangeable in all models of Thompson guns.

There are 3 styles. The 1921/1928 1-piece grip mount has an upward bend which creates a great deal of interference with the barrel. When it is installed it is pre-stressed to provide the rigidity required by the vertical foregrip. The 1928A1/M1 grip mount is the same as the 1921/1928 except that they do not have the upward bend. The bend is not necessary since they are used only with horizontal foregrips which bear directly against the barrel. M1A1 grip mounts were made from 3 pieces riveted together. They are therefore referred to as the 3-piece grip mount.

The 3-piece design was a failure because the rearmost riveted part required a hole in the body of the grip mount near the front end of the receiver where the grip mount needed its maximum strength. A downward pull on the sling can bend the grip mount downward, away from the barrel. To solve this problem, a strap was fitted clamping the barrel and front end of the forearm together. This is a classic case of a penny saving change costing dollars to fix.

                                                          Firing Pin Springs

QUESTION : Are the firing pin springs identical for all models of TSMGs?

ANSWER: Models 1921, 1928 and M1 TSMGs all have separate firing pins using a firing pin spring. The Model M1A1 does not have a separate firing pin and so does not use a firing pin spring. TSMG firing pin springs are identical at 32 coils total. However, there was a time during the development of the 1928 model that some 1921 springs were shortened 2 coils for use in the 1928. Therefore, some guns, particularly 1928 Navy models, may be found to have springs with only 30 coils.

                                               Shooting A Colt

QUESTION : I own a 1921 Colt TSMG. I have been told not to shoot it as something might break and that would destroy the value of the gun. Should I shoot it or not?

ANSWER: That is the dilemma. Other than physically damaging the gun by dropping it or something like that, there are only a few things that can break. Bad ammo may result in a bullet being stuck in the barrel. Several more bullets will enter the barrel before the problem becomes obvious. The barrel will probably bulge and be ruined. A bad cartridge may separate upon firing, causing the bolt feed lip to break off, the extractor to break and, possibly, the ejector will break. Even with no outside causes damaging the gun, one of the “ears” on the actuator may break off. My belief is that this is caused by microscopic cracks resulting from the heat treating process. After a period of pounding from shooting, the crack propagates through the metal and off goes the “ear”. There is nothing that can be done to prevent that and no acceptable way to fix it.

Therefore, I say go ahead and enjoy shooting the gun, but replace the parts subject to breakage with WW2 and/or aftermarket parts before unrepairable damage is sustained.

                                                   Trip Projection

QUESTION : What does the projection on top of the front end of the trip do?

ANSWER: Nothing. There are three styles of trips. The first two have the projection. There is a corresponding ramp on the bottom back end of 1921-M1 bolts. It would seem that the ramp forces the trip downward at the front end as the receiver and frame (trigger housing) assemblies are mated. But it is not necessary to do that. Finally, the M1A1 gun deleted the trip projection and bolt ramp. Since the M1A1 trip works perfectly in all Thompson models, the trip projection solved a problem that never existed.


                                                          Lefty M1/M1A1 Bolt Handle

QUESTION : I am left handed and wish the M1/M1A1 bolt handle had been located on the left side of the gun. Would that have been difficult to do?

ANSWER: I am right handed and I wish the M1/M1A1 bolt handle had been located on the left side of the gun. It seems that only the U.S. Army likes bolt handles on the right side of guns. Most everyone else prefers the bolt handle to be on the left side or on top. In the case of the Thompson, a bolt handle on the right side means the shooter has to release his hold on the pistol grip (for us righties) in order to cock the gun - a real time waster! Since the 1921/8 bolt handle is on top, this problem does not exist on those guns. And, there is no reason the M1/M1A1 bolt handle can not be on the left side of the gun. If there were a bolt handle slot on the left side of the receiver, the bolt handle could be installed out the left side with no other changes. As a matter of fact, at least one M1/M1A1 gun was manufactured with bolt handle slots on both sides of the receiver. I assume it was made to demonstrate the possibilities. In any case, the decision was made to locate the handle on the right side. I have addressed this problem by offering my M1/M1A1/M2 style Display (TM) receivers with the bolt handle slot(s) anywhere the buyer wants it (them).


                                                                       Bent Actuators

QUESTION : I have seen actuators which are bent in the lock slot area. Is there any way to prevent that from happening?

ANSWER: This problem is usually caused by a worn lock which enables the actuator to go too far forward in the bolt so as to be stopped by the hammer. The actuator then rides up on the hammer causing it to bend. The solution is to check the bottom front edge of the actuator for a wear spot in the center. If the bluing is worn, it is a sure sign that the lock is worn and actuator damage is coming.




                                                              TSMG Barrel On N/K Gun

QUESTION : What is involved in changing the standard 16" barrel to a TSMG barrel on a Numrich/Kahr gun, assuming that the proper paperwork and tax payment has been completed?

ANSWER: The N/K gun differs from the TSMG in several areas that involve the barrel. The N/K barrel has a smaller diameter and a different profile than the TSMG barrel. The undersized diameter causes problems when using my barrel wrenches and so it must be either removed by holding it in a vise between wood blocks or wrapped with card stock to increase the diameter. The different profile and N/K 3/4" wide grip mount means that the grip mount and forearm/grip must be replaced with Thompson parts. Since the compensator is fixed to the barrel with adhesive and not pinned, it may be necessary to heat the compensator to break the adhesive. If a ring sight is fitted, it may have to be replaced with a TSMG sight as it may not fit properly on the TSMG barrel. Otherwise, standard Thompson barreling procedures should be followed.


                                  Interchanging Thompson Rear Sights

QUESTION : Can one style of Thompson rear sight be interchanged with another.

ANSWER: Yes. All Thompson rear sights are interchangeable. They are all located in the same place on all models of Thompson guns, have exactly the same hole pattern and use the same special Thompson rear sight rivets. The rear sight rivets are installed up through the receiver and then through the sight. Although, the M1/M1A1 receiver is the same width as the Lyman adjustable rear sight, if that sight is installed on an M1/M1A1 receiver, it will appear to overhang on the sides because of the top edge radii of the receiver.



                                            Make A Machine Gun

QUESTION : I bought a Thompson parts kit and now I want to buy a semi-finished machine gun receiver and have my local machine shop finish the receiver. Where do I go with this?

ANSWER: Probably to jail along with the machinist. Only a holder of a Class 2 manufacturing license can legally make a new machine gun. I am not an authority on the paper work required. The Small Arms Review magazine deals with the whole area of machine gun manufacturing and sales so that might be a good place to get information.

                                                            Gun Info

QUESTION : I bought a 1928 Thompson parts kit with serial number XXXXXX. Can you tell me who made the gun and when?

ANSWER: If it is a 1928, it was probably made in 1941. If it is a 1928A1, it was probably made in 1942 or 1943. If the barrel is not finned, it would be one of the last ones made. Both Savage and Auto-Ordnance made the guns with Savage prefixing their serial numbers with ”S” and Auto-Ordnance prefixing with “A.O.”. I know of no records being available which would provide more information.


                                                   RECEIVER STEEL

QUESTION : Unfinished receivers are being offered in many kinds of steel. What type is best?

ANSWER: Occasionally I get a call asking what type of steel I use to make my receivers. I get interested because I think we are about to have a discussion about metallurgy so I will usually ask what type of steel the caller would prefer. The response I usually get is that the caller knows nothing about steel types. That makes me wonder why the question was asked and the question is answered in my catalog anyway.

But the question asked here is different. When an engineer selects a steel alloy to use to make a product, there are several things to consider including strength, machinability, availability and cost. Strength is determined by the steel’s ability to be hardened. The ability to be hardened and the characteristics of the hardened steel are determined by the alloying elements added to the iron to make the steel. Steel can be machined in its annealed (soft) state and heat treated later or machined in it’s hardened state to begin with. The trade off is that heat treating steel to harden it shocks the steel and always ends up with some distortion. Thompson receivers were originally made from several types of steels which became obsolete over the years. The last Thompson receivers were made from type1141 and then heat treated. Any steel that meets the Thompson specifications for strength can be used but keep in mind that strength is not the only factor. Will the receiver twist during subsequent machining operations? Will it be a problem to machine additional features? Has it been stress relived or is it like a coiled spring ready to move when surrounding metal is removed? So, “best” can be argued. I believe that they selected the best steel alloy and strength originally and that millions of guns used in WW2 proved it. The only change I make is to machine the receivers from the steel in it’s hardened but stress relieved state so there is no subsequent heat treating to cause it to twist during heat treating or during subsequent machining.



                                              1921 Serial #1

QUESTION : Does any one know where 1921 Colt Thompson serial #1 is?

ANSWER: There never was a serial #1 or for that matter serial #2 through 40. Colt never contracted with the Auto-Ordnance Corp. to manufacture 15,000 Model 1921 Thompson guns. Colt did contract to make 15,000 Model 1919 Thompson guns starting with serial number 41 to pick up where Auto-Ordnance left off with their own production. Colt serial #41 was, in fact, a Model1919, but that gun was immediately modified and reconfigured to such an extent that the model name was changed to 1921. So the first Model 1921 gun was serial number 41. The mystery is why Colt didn’t mark the next 40 guns #1 through 40 so that the serial number range would have started at #1.

Interchanging Barrels

QUESTION : I tried to install a used barrel with front sight on a new receiver and the sight did not end up in the vertical position. What is wrong?

ANSWER: Nothing is wrong. Thompsons do not have a qualified thread (also called ordnance thread) which is typical of most guns. The front sight is installed after the barrel has been installed. There is no reason to assume that another receiver will have exactly the same thread position because there is no specification for it. This is probably the biggest problem encountered by persons trying to use a parts kit with a new receiver. Unfortunately, because of the coarse pitch of the barrel thread, it is not possible to further turn the barrel once it is tight. The sight must be moved.

                                               Thread Sizes

QUESTION : What is the correct thread size for all the threaded Thompson parts which appear to be about 1/4" in diameter?

ANSWER: #14-24. #14 is .242" diameter. The Thompson uses the same size thread for everything except the compensator, barrel, buttplate door, buttstock reinforcement, 1927 & 1928 (Colt) sling swivels and adjustable rear sight. This is a thread size that is seldom used today. 1/4" (.250") has become the standard but back in 1920 that was not the case. This does not create any particular problem except when someone assumes the thread is 1/4" and so ruins a part by retapping it oversize. Of course, finding taps and dies in #14 size is difficult.

                                                             Sling With Vertical Foregrip

QUESTION : What is the best way to fit a sling to a Thompson with a vertical foregrip?

ANSWER: All Model of 1921 and all Model of 1928 (Savage) guns were made with vertical foregrips. Colt guns had no provision for a sling at all whereas the Savage guns did have a sling swivel on the buttstock. During WW2, the British Commonwealth Countries added a sling swivel to the side of the foregrip in order to fit a sling but this was never a factory fitment. In 1922, Auto-Ordnance introduced the Model of 1922 which had a Springfield style swivel retrofitted to a standard 1921 buttstock. A horizontal forearm was introduced with the 1922 which incorporated the second swivel. The swivel equipped buttstocks must have inspired A-O to come up with a way to fit a second swivel on standard 1921 guns so a sling could be used. In 1922, A-O offered 1921 guns fitted with slings as an option. They offered a barrel band which fitted around the barrel just ahead of the foregrip. Unfortunately, this option must not have been popular because no example (or even a photo) of a factory fitted sling on a 1921 gun is known to exist today. In my catalog, I offer an entire sling system for all Thompson guns that includes sling barrel bands and special sling pilots. How close I came to the 1922 barrel band design is only a guess.


                                                          1921 v. 1928 Bolts

QUESTION : What is the difference between a 1921 bolt and a 1928 bolt or is there any?

ANSWER: Yes, there is a difference. All 1921 bolts are nickle plated. But so are some 1928 bolts. 1928 WW2 bolts are marked with the manufacturers symbol whereas Colt bolts (1921) are unmarked. The real difference is in the way the inside “U”-shaped edge on the back end of the bolts are radiused. 1921 bolts have a circular countersunk depression that bevels the centers of the three sides of the “U”. That pattern matches the round collar on the metal portion of the 1921 Buffer. 1928 bolts have the entire edge of the “U” radiused just to eliminate sharp edges which has no relationship to the larger and flat collar of the 1928 pilot. 

                                                                   Gripmount Styles

QUESTION : How many styles of gripmounts were made and how do they differ?

ANSWER: There are three different styles of gripmounts that were fitted to Thompson guns. The first style was fitted to Models 1919, 1921, 1922, 1923, 1927, 1928 and Colt 1928A1 guns. It is a 1-piece gripmount in that it was machined entirely out of one piece of steel. This gripmount is bent up slightly such that the barrel pushes it downward with considerable force. The reason for stressing it was to create a very rigid mount for the vertical front grip that was originally fitted to all 1919 and Colt guns. When the WW2 production switched from the Savage 1928s made for Great Britain, which had vertical front grips, to the U.S. 1928A1s which were fitted with horizontal forearms, there was no longer a need for the rigidity. That lead to the elimination of the bend and the introduction of the second style of gripmont. In my opinion, the projection at the front end of the gripmount which bears against the barrel should have been eliminated completely. This is because it is the forearm which bears against the barrel as the forearm screw is tightened. As the screw is tightened, it is actually pulling the gripmount away from the barrel such that, in most cases, the gripmount no longer contacts the barrel at all. The third style is referred to as the 3-piece gripmount and fitted to M1A1 guns,. This gripmount consists of a flat bar fitted with a front barrel stud and a rear securing stud. Both studs are actually rivets so the 3-piece gripmount can not be disassembled. This gripmount was a complete disaster. It was supposed to save money but, in fact, it is so weak because of the rear stud hole reducing its strength, that it failed in service. By pulling downward on the sling, it is possible to bend the gripmount. To solve this problem, a strap was designed to go around the barrel and forearm and retro-fitted to many of the guns. Later Army drawings show a fourth style that is similar to the U.S.1928A1 style in that it is a 1-piece design but differs in that the front projection is replaced by simply bending the front end upward. There is no evidence that any of the fourth style were made.


                                                             M1: Improvement ??

QUESTION : The M1 Thompson was cheaper and simpler compared to previous models, but was there anything about the gun that was an improvement?

ANSWER: Cheaper and simpler pretty much sums it up. But, yes. There is one improvement that was incorporated in the M1 gun. It is an additional sear slot near the back of the bolt that enables the safety to work when the bolt is closed. On previous models, the safety will only work if the bolt is cocked open. Soldiers during WW2 tended to keep the bolt closed to prevent dirt from entering the breech. If the bolt were closed and the gun were dropped on its butt, it would be possible for the bolt to cycle and fire a cartridge. So, the addition of the closed-bolt-safety-slot was definitely an improvement. Earlier bolts can be modified to incorporate this safety feature. If the guns had been made today, they would probably all be recalled to add it.


                                           Kinked 1928 Recoil Spring

QUESTION: What is the cause and cure for the kinking of a1928 recoil spring?

ANSWER: I have often said that there is no engineering justification for the existence of the 1928 Thompson gun and that every change made to the 1921 Thompson design in order to create the 1928 degraded the performance and characteristics of the 1921. The major case in point being the 1928 recoil spring and pilot design. The only purpose of the pilot is to hold the spring in place in the space between the rear end of the receiver and the back end of the bolt/actuator. Without the pilot there would be nothing to prevent the spring from looping out at its side and kinking. But the pilot is not long enough to fully guide the spring over the entire space. This is because the pilot can not be made long enough to completely guide the spring and still be able to be installed from the bottom of the receiver. On the 1921 gun, the non-piloted gap was not a problem because the diameter of the recoil spring is greater than the length of the gap and so there are not enough unguided coils to loop out. The 1928 gun design reduced the diameter of the spring and increased its stiffness to the point where it is always on the verge of looping out and kinking and will do so if there is any additional aggravating factor. Assuming that the gun is an original 1928 Thompson, which would eliminate such contributing factors as an oversized pilot hole in the receiver, an undersized diameter and/or length pilot or a buffer that is too thin, then it is only necessary to check that there is a breech oiler and buffer in place. If either of those are missing, the gap is increased. Given that the gun is in specification configuration, the cause of spring kinking is the result of operator error in installing the spring. With the spring and pilot out of the gun, as much of the spring as possible must be forced onto the pilot and then secured in place by a pin pushed between the spring coils and through the hole in the front end of the pilot from the bottom side. A 1928 Recoil Spring Stripping Tool, pin punch or even a nail can be used for this purpose. This will allow the end of the recoil spring to be easily fed into the actuator hole and the pilot (with buffer in place) fitted through the oiler and receiver holes with no possibility of looping and kinking. Once the spring/pilot assembly is in place in the receiver, remove the tool. If this procedure was not previously followed and the spring kinked during installation (or because of any other problem), it should be replaced.


                                                        Barrel Removal

QUESTION : How does one remove a Thompson barrel, particularly from a parts kit receiver nose section?

ANSWER: Actually, removing a Thompson barrel is quite easy as long as the barrel is to be scrapped. If the barrel is to be saved, which is invariably the case today, there is just no easy way. The two worst aspects of the Thompson design are the 1 in 10 pitch square barrel threads and the grip mount not being removable without first removing the barrel. There is no excuse for the barrel thread design. It is difficult to manufacture and gage and requires tremendous force to tighten or, especially, loosen the barrel. Although Auto-Ordnance came up with several alternative grip mount designs that enabled the grip mount to be removed without removing the barrel, apparently, none proved to be as good as the one used. The result is that the grip mount is always in the way of trying to remove an already problematic barrel. The best advice I can give is to refer to an article I wrote entitled “Barrel Removal”.

                                               Barrel Removal

According to Army Technical Manual TM 9-1215 on the Thompson Submachine Gun, “To remove barrel, disassemble the gun, wedge a block of hard wood in receiver to prevent springing of the side, clamp receiver in a vise with leather jaws and unscrew the barrel from receiver, using a strap wrench. If a barrel is to be scrapped, a pipe wrench may be used.”

Follow those instructions and you will very likely destroy your gun. The receiver is just not strong enough to withstand the twisting force of barrel removal when held in a vise as described. And no block of hardwood will prevent a vise from collapsing the sides of the receiver. The writer of those instructions was most likely relying more on imagination than experience.

Some barrels, particularly on World War II guns which have seen abuse, are very stubborn and refuse to yield to anything but great force. When that situation is encountered, there is little possibility of removing the barrel without proper equipment. Other times, the barrels may be little more than hand tight.

There are two approaches to removing the barrel: 1. Hold the receiver and turn the barrel or, 2. Hold the barrel and turn the receiver. The method chosen is primarily based on the tools and equipment available. In the home shop and without professional tools, it is best to hold the barrel and turn the receiver. This is because it is easier to hold the barrel in a vise than to grip it with a wrench. Holding the barrel is the main problem. Strap wrenches tend to slip or crush the fins on finned barrels. A pipe wrench does work well but goodbye barrel.

To begin, strip down the gun to the bare receiver/barrel/grip mount assembly. Using the biggest wrench vise available, grip the barrel in the vise jaws between blocks of soft wood like pine. The receiver assembly must be upside-down with the vise jaws close to the receiver. With the receiver upright in the vise. Note that the grip mount is trapped between the vise jaws/blocks with very little clearance on either side. Since the grip mount is attached to the receiver, it will turn with the receiver as the receiver is turned. It is obvious that before the receiver has turned very far, the grip mount will be stopped by the block and will be bent. Therefore position the receiver assembly so that the grip mount is above the blocks and next to the block away from which the grip mount will be turning as the receiver is turned. Tighten the vise. In the case of finned barrels, the wood blocks should go all the way to the base of the fins with the fins cutting into the blocks.

Grasp the receiver nose with a large adjustable open end wrench with a piece of leather between the receiver and wrench in order to protect the receiver. Tighten the wrench to avoid having the wrench engage the corners of the receiver nose. NEVER GRIP THE RECEIVER ANYWHERE OTHER THAN BY ITS NOSE END. With luck the receiver can be unscrewed approximately 1/8 of a turn before the grip mount contacts the other block. It may be necessary to reposition the receiver/barrel/grip mount assembly in order to gain another 1/8 turn which should be sufficient. If the barrel will not turn, it is sometimes helpful to soak the receiver/barrel assembly in gasoline over night. If a parts kit cut-off receiver nose is involved, the receiver nose can be heated red hot with an acetylene torch. Care must be taken not to apply the flame to the barrel or grip mount and not to allow those parts to be over heated. Let the assembly completely cool before again trying to unscrew the barrel. Some barrels will resist all efforts toward removal without special tools. Thompsons have a 1 in 10 square (not acme) thread that is a fairly loose fit, so a 1/4 turn of the barrel should loosen the barrel sufficiently for hand removal. A strap wrench can be used at this point because the force applied to the barrel will be minimal. Remove the assembly from the vise.

Screw a horizontal foregrip screw into the grip mount. Position the claw of a carpenter’s claw hammer around the grip screw as though the screw were a nail to be removed. Pull back on the hammer just enough to force the grip mount away from the barrel and unscrew the barrel by hand.


                                                     Barrel Torquing Specification

QUESTION: What is the proper torque requirement when installing a Thompson barrel?

ANSWER: There is no torquing specification that I am aware of and it would not be relevant if there was one. There must have been some figure used at Savage and Auto-Ordnance when the guns were made. It would only make a difference if very powerful and expensive machines were used, and they must have had them. From a practical stand point, it is virtually impossible for an individual to over tighten a Thompson barrel. This is because of two factors. The 1 in 10 pitch of the Thompson square barrel thread does not allow any “tweaking”. The barrel goes from loose to tight in just a few degrees of rotation. The barrel does not get tighter and tighter as is the case with V-threads where the external part (nut) actually expands as the tightening force is increased. If it were possible to measure the torque it would go from 0 to off the scale instantly, making a meaningful measurement unobtainable. The other factor is that it is impossible with reasonably available tools to hold the barrel tight enough to be able to over tighten it. Getting the barrel as tight as possible becomes the goal

                                          Worn Barrels

QUESTION : How does one know if a Thompson barrel is worn out?

ANSWER: When the lands and grooves are no longer distinct, the barrel is worn out. Another indication is that shooting accuracy has fallen off. However, very few Thompson guns will reach the point where the barrel is worn out if military specification barrels are fitted. Unfortunately, most new-made barrels are not made to military specifications, so wear may be more of a problem in the future. A much more likely problem will be bore damage. WW2 guns will often show the affects of corrosive ammunition. This is indicated when the bore appears to be worn (actually eroded) at the breech end rather than over its full length. What is a more worrisome possibility is a ringed or bulged bore caused by defective ammunition. Being a machine gun, a bullet that does not make it out the end of the barrel is immediately follow by more bullets that jam in the barrel, destroying it. At the prices Thompsons are selling for, damaging a barrel becomes a catastrophic loss. Therefore, if a nice condition, all original gun is to be shot, I recommend replacing the barrel assembly so that there is no chance of damaging the original one.


                                         Ring Front Sight On Threaded Barrel

: Can a standard ring style front sight be installed on a barrel that was threaded for a compensator?

ANSWER: No. The muzzle of a barrel made to accept a ring style front sight is larger, smooth and tapered compared to the straight threads on the muzzle of a barrel made to accept a compensator. It is possible to thread the end of a ring sight barrel to accept a compensator. If a ring sight is put on a threaded barrel, it will not be a tight fit. Of course, it could be welded or epoxied to hold it in place, but that is redesigning the gun. I have had several inquiries about the possibility of making some ring sights threaded like a compensator so there would be interchangeability, and that could be done. The problem is that there is just not enough demand to make such a product viable.


                                     Parts Kit Semi-Auto

QUESTION : What is involved in making a semi-auto with my TSMG parts kit?

ANSWER : Unfortunately, to the best of my knowledge, as of this date (Jan. 2007) there is no ATF approved way of making a semi-auto gun from a TSMG parts kit as is done with a number of other machine gun parts kits. The assumption being that the semi-auto gun will have the outside dimensions of the original, look exactly like the original and use most of the original parts including magazines. Numrich Arms designed a semi-auto gun years ago that more or less resembles a Thompson. That gun is now manufactured by Kahr Arms. Neither of these companies has any connection to Col. Thompson’s Auto-Ordnance Corp. that invented, developed and manufactured the Thompson Submachine Gun. The N/K gun is not a true Thompson and not made to the original drawings. Therefore, very few Thompson parts fit it. Without an approved design to modify the TSMG to fire semi-automatically, the only option is to make an N/K gun out of a Thompson. Unless one is willing to spend a lot of money and register the gun as a short-barreled rifle, it is not a cost effective project. (My book, “Thompson New Production Semi-Autos” describes this process.) One is better off just buying the N/K gun. If a detachable buttstock is desired, the most cost effective way of achieving that is to fit one of my kits to the trigger housing. If it is desired to have a receiver that is as true to the original as possible, then one of my MS or 2S receivers can be used. But in the end, it still ends up as an N/K gun design, not a Thompson. I have a design for a proper semi-auto Thompson but no time to pursue it. I know others are working on their versions. Maybe something will come of it.


                                                    ALUMINUM TSMG

QUESTION :  Why was there never a TSMG made of aluminum?

Savage did manufacture a Model of 1928 Aluminum TSMG. It had an aluminum receiver and trigger housing left in the white. The buttstock and grips were brown plastic. All other parts were standard black steel. The WW2 Model of 1928 Thompson guns were made for the British including the aluminum version. But, strangely, since all the guns made previously for the British had vertical foregrips, the aluminum gun was fitted with a horizontal forearm. Unfortunately, the aluminum did not pass British testing. It wore excessively on the magazine slots and the receiver was not strong enough. Only one complete original gun is known to exist today and it belongs to the British Government. Had an aluminum M1 been made, it may well have proven to be acceptable. That is because the back end of the receiver is much stronger than that of a 1928 model. Also, since the model M1 will not accept a drum magazine, the stress a drum puts on the receiver slots would not be a factor


                               1921/8 Safety Lever Handle Hole

QUESTION : What is the purpose of the hole in the blade handle on safety levers fitted to 1921. 1928 and early 1928A1 Thompsons?

ANSWER: The only purpose of the hole is to provide an axis for the knurling tool as it traveled around the circumference of the radius at the end of the safety lever. When the knurling was eliminated on late 1928A1 gun production there was no longer a need for the hole. Although the rocker pivot (selector) was also knurled, the knurling did not wrap around the lever to the extent that it did on the safety. Therefore, a different setup was possible for the knurling tool that did not require an axis hole in the lever.


                                                M1A1 Bolt Handle Groove

QUESTION : At the center of the shank portion of the M1A1 bolt handle (the part that goes into the bolt) there is a groove which lines up with the recoil spring. Does the groove have something to do with the spring?

ANSWER: No. The groove has nothing to do with the spring. The fact is that the groove has nothing to do with the M1A1 gun at all. The A M1A1 bolt handle@ is really not an M1A1 bolt handle. It is an M1 bolt handle which was carried over for use on the M1A1 probably to avoid having to stock two types of bolt handles. The purpose of the groove on the M1 gun bolt handle is to limit the rearward movement of the hammer. The bolt handle shank groove diameter determines the hammer movement limitation. On the 1921/8 gun, the back of the hammer slot in the bolt provides the hammer stop.

                                  1928 Bolt Ramp

QUESTION : There is a ramp (inclined groove) on the bottom back end of the 1928 bolt. What does it do?

ANSWER: You could also have asked the same question about the projection at the top front of the trip because the answer would be the same - nothing! My guess is that it must have seemed necessary to make sure that the trip was in the downward position as the receiver assembly was mated with the trigger housing assembly because that seems to have been the only possible purpose of the ramp and projection. (i.e., the projection is pushed down as it engages the ramp.) This was a solution to a problem that never existed. Someone must have figured this out during the design of the Model M1 gun because the trip projection was eliminated even though M1 bolts still have the ramp. The trip was further simplified and the ramp eliminated with the introduction of the M1A1 gun. The M1A1 trip is the best design of the three versions. The proof of the uselessness of the projection is that an M1A1 trip can be installed on any model Thompson gun with no effect.

                                       M1A1 v. M1 TSMG RELIABILITY

QUESTION: It is my understanding that the M1A1 TSMG is considered to be more reliable than the M1 because of the elimination of the firing pin, firing pin spring, hammer pin and hammer pin. Is that true?

ANSWER: There is no question that fewer parts to break, wear or foul, usually results in more reliability, but that is a minor factor with the Thompson which never suffered from a reliability problem. The main reason the M1A1 gained increased reliability was that the bolt shank (the cylindrical part of the bolt which actually enters the breech) was made longer which resulted in the bolt being stopped by the back of the cartridge rather than by the front end of the receiver bolt cavity. That coupled with the firing pin being machined into the bolt face, which guarantees penetration of the firing pin into the primer, helped eliminate misfires due to defective cartridges.

                               INTERCHANGING TRIGGER HOUSINGS

QUESTION: I have been told that all TSMG and Numrich/Kahr 1927 type trigger housings are interchangeable on all TSMG and N/K receivers and I have also been told they are not. Please clarify this.

ANSWER: All TSMG trigger housings will fit all TSMG receivers regardless of model. All N/K trigger housings will fit all N/K receivers regardless of model. However, an M1/M1A1 (either TSMG or N/K) trigger housing will not accept a drum magazine unless it is modified. There will also be a mismatch between the rear ends of the receiver and trigger housing when M1/M1A1 and 1921/8 style parts are mixed. An M1/M1A1 TH rear end can be modified to match the rear end of a 1921/8 receiver but a 1921/8 TH can not be modified to match the rear end of an M1/M1A1 receiver. The 1921/8 TH mismatch is, however, hidden underneath the M1/M1A1 receiver and so is not easily seen.

A TSMG TH will not fit a N/K receiver without being modified by cutting a slot across the front. TSMG TH internal components will not fit in a N/K TH and will not work with a N/K bolt assembly. N/K components can be fitted in a TSMG TH with slight modifications. A N/K TH will fit on a TSMG receiver but will not work with the TSMG bolt. Probably no one would want to do that anyway.


                                  MODELS  OF 1922 & 1923

QUESTION: I have never seen a reference from the time period mentioning the Model of 1922 designation. I have always thought that the different variations of the few known Model of 1923 Thompsons were only prototype variations of a failed product. Please clarify this.

ANSWER: Neither the Model of 1922 nor the Model of 1923 are known to have been mentioned anywhere in the original documentation. Auto-Ordnance called everything “the Thompson gun” which lumped Model of 1919 and Model of 1921 together. In 1922, A-O tried to boost their meager sales by coming up with something along the lines of the BAR in order to appeal to military organizations. The guns they produced were categorized as “The Thompson Gun - Military Model” with no differentiation between actual models, and of course, did not change the guns original “1921" marking. The gun that was shown in the 1923 Catalog was not called a Model of 1923, but since it appeared in the 1923 Catalog, collectors referred to it as the “Model of 1923". What collectors did not realize was there was another “military” model which came before it. There has only been one vintage photo of it found and it was not a good picture. The only example of the gun was thought to be in the West Point Museum but it is not complete and had been extensively modified toward the style of the 1923. Thompson students, including myself, assumed that it was a tool room prototype of the 1923 and it is. What we all failed to realize was that it was only one of the 1922s that had been made and it had been retained by A-O to use as a basis for the development of the 1923. I initiated a quest to find a 1923 in order to document it completely and reproduce it. I have never found one. What I did discover was that only one was documented to have been made. It never left A-O and was eventually broken up for parts. What I did find was a number of intact1922s but since the common belief was that it did not exist, it had no name. So I named it based on the fact that it had to have been made in 1922 (or possibly into 1923). I also refer to both the 1922 and 1923 as “Thompson Submachine Rifles” in keeping with the nomenclature at the time. Guns that collectors are claiming to be 1923s to enhance their value are nothing more than standard 1921s fitted with left over 1922 barrels.

                                     DRUM ON M1

QUESTION: Why can’t an M1/M1A1 receiver be modified to accept a drum magazine or can it?

ANSWER: An M1/M1A1 receiver can not be modified to accept a drum magazine because so much of the receiver would have to be cut away to allow the drum to slide in from the side that the receiver would be unacceptably weakened. Some people have used drums on M1/M1A1 guns by fitting a “T” shaped bracket to the back of the drum so that it loads upward on to the gun like a box magazine.

                         SAVAGE & A-O TRIGGER HOUSINGS

QUESTION: What is the difference between M1 & M1A1 trigger housings made by Savage and those made by Auto-Ordnance?

ANSWER: M1 and M1A1 trigger housings made by Savage differ from those made by Auto-Ordnance in three primary areas. They are marked differently. Savage TH are marked “FULL” on one line and “AUTO.” on the line below. A-Os are marked “FULL AUTO.” on one line. They also differ in the way the metal is shaped behind the grip. Savage THs have a constant curve across the TH whereas the A-Os have a flat surface with the edges radiussed. And only Savage THs have the “GEG” acceptance mark. Disregarding these and other slight manufacturing differences, they are identical and completely interchangeable.


                             RECEIVER/TRIGGER HOUSING END MISMATCH

QUESTION: I purchased a non-Richardson semi-finished receiver and found that there was a large mismatch between the end of the receiver and the end of the trigger housing. It seems that the receiver is too much longer than the trigger housing than it should be. What would be correct?

: The receiver is designed to be .002" longer than the trigger housing. However, combining the plus and minus tolerances (errors) allowed in the manufacture of both the receiver and trigger housing in order to determine the extremes of allowable mismatch (i.e. the longest allowable receiver mated with the shortest allowable trigger housing and the shortest allowable receiver mated with the longest allowable trigger housing), the receiver can be .014" longer than the trigger housing or the trigger housing can be .024" longer than the receiver. As a design engineer, I think the tolerance that allows the trigger housing to extend beyond the back of the receiver is a poor design because it is ugly. The science of tolerance and an appreciation of its effect on the product is often not understood by designers. Because I deal with Thompson fanatics (not unlike myself) who insist on having good fits and because I use very expensive and sophisticated machinery in the making of my Thompson Display receivers and other parts which makes it possible, I ignore the maximum allowable tolerances and make my parts to substantially closer tolerances than the specifications allow.

                                A5 BARREL THREADS

QUESTION:   Are the barrel threads on a Numrich/Kahr A5 Thompson Pistol the same as a TSMG?

ANSWER:   Theoretically, they are. But the N/K gun is not a Thompson and so the quality and conformance to Thompson specifications of the threading leaves something to be desired. You did not say what you are intending to do. If your intention is to install a 10˝" TSMG barrel on an A5, you must register it as an NFA weapon. Mere possession of an A5 gun and a TSMG barrel would probably be considered to be a violation. The best thing to do with an A5 is to leave it alone and not have any parts around, including an N/K 1927A1 gun, that could be used to assemble an NFA weapon. This confusion is caused by the A5 being marked “pistol” when it is not classified as a pistol by the ATF.

                              COLT SLING SWIVEL

QUESTION What would be the correct type of sling swivel on a Colt TSMG?

ANSWER:  Colt never put a sling swivel on a TSMG during the Model of 1921 gun production. In 1922, Auto-Ordnance fitted 1921 Colt buttstocks with 1903 Springfield Rifle swivels for the Model of 1922 Thompson Submachine Rifles. In 1923, entirely new buttstocks designed without any drop at the rear end and newly designed horizontal forearms were made and again fitted with Springfield swivels for fitment on the Models of 1923 Thompson Submachine Rifles. In 1927, 1921 Colt buttstocks and redesigned forearms were fitted with offset, 3-piece Enfield swivels for the “new Colt” Models of 1927, 1928, 1928 Navy and 1928A1. In 1936, A-O went back to using the Springfield swivels. It is interesting that around 1923, A-O offered Model of 1921 guns fitted with slings. This was done by fitting 1922 buttstocks and either 1923 forearms or fitting a sling barrel band in front of the standard vertical foregrip. Unfortunately, there is no known example or even a photo of the sling barrel band. I do offer my own version in my catalog.

                             WW2 TSMG FINISH

QUESTION: Some people claim that WW2 TSMGs were Parkerized and others claim they were black oxided. Which is true?

ANSWER: No TSMG ever left the factory with a Parkerized (phosphate) finish. All WW2 TSMGs were black oxided. The confusion comes from 3 sources. Mostly, people don’t know what they are talking about. But, a lot of guns were refinished (not by the U.S. Army) with a phosphate finish because it hides a lot of imperfections and it is more durable than black oxide. The last source of the confusion comes from the U.S. Army. All the Thompson drawings were redone in the 1950's by the Army and those revised drawings specify phosphate. So, if the Army were to put the Thompson back into production, the new guns would be phosphated. However, the Army seems to have lost all the Thompson drawings and they never liked the Thompson anyway.

                         M1 & M1A1 TRIGGER HOUSING SERIAL NUMBERING

Are all M1 & M1A1 trigger housings serial numbered?

ANSWER: M1 and M1A1 trigger housings made by Savage are serial numbered on the bottom of the trigger guard whereas the ones made by Auto-Ordnance are not serial numbered. However, sometimes new serial numbers were stamped on for various reasons by military depots, etc., which confuses the issue. Those numbers are usually recognizable by their placement or crudeness.


QUESTION:  Did Auto-Ordnance ever make a vertical foregrip screw with a sling swivel on the end? I have seen one like that.

ANSWER:  No. Things like that are aftermarket creations by some individual or sometimes they are movie props. I am located in the vicinity of several movie prop shops which are prolific at coming up with novel Thompson ideas that look great on the screen but would never hold up in real service like quick detachable barrels, folding stocks and quick removable gripmounts. In this case, I can’t imagine how such a sling swivel could work. Interestingly, someone wrote an article about a “rare” Thompson barrel that he had found at a gun show. It accepted a Sionics silencer. It turned out that it was something I had made and offered years ago in an old catalog. What amazed me was all the speculation and assumptions that were made about it as to its probable Thompson history. It had no history.

                      1928 & M1/M1A1 RECOIL SPRINGS

QUESTION:  The 1928 recoil spring is U.S. Army part number A153024. The M1 and M1A1 recoil spring is A153116. But, I am told the 1928 gun and M1/M1A1 guns use the same recoil spring. How can this be?

ANSWER:  Spring A153024 is identical with spring A153116 so, the answer is that the 1928 gun and M1/M1A1 guns use the same spring. This confusion is caused by the Army using the drawing number as the part number. The part is the same but the application (spring loading, etc) is different for the different guns. The application specifications require different drawings even though the part is the same. The problem comes at the depot level when someone needs a replacement spring for a 1928 gun but all they have are spare springs for the M1 and M1A1 guns. The box for the 1928 springs is empty. How is the parts guy supposed to know they are the same if the part numbers are different? Drawings are engineering tools. Part numbers are assembly tools. The two should not be mixed. Fortunately, this is the only Thompson part that I can think of where this situation exists.

                       TSMG WON’T FIRE IN SEMI-AUTO

QUESTION:  When I fire my Thompson in full-auto, it works perfectly. But, when I switch to semi-auto, it fires once and then the trigger remains jammed in the rear (pulled) position. If I pry the trigger forward into its normal position, the gun will again fire once with the same result. What’s wrong with my gun?

ANSWER: This problem is caused by dirt and/or bad or wrong spring(s). It would seem that it is time for that 50 year service on your Thompson. Disassemble your gun and remove and break down all trigger housing components. Clean out all spring holes with twist drills the same size as the holes. Clean everything with gasoline. Check and replace all springs as needed. Reinstall all springs making sure they bottom out in the holes and don’t interchange the disconnector and sear springs. Reassemble the trigger housing. Lightly oil the components. Check for correct operation of the trigger hosing assembly. Reassemble the gun. Repeat this procedure in 50 more years. In the meantime, don’t mess with the gun. It was taking the trigger housing apart that probably caused your problem in the first place. A quick gasoline wash and light oil once in a while without disassembling the trigger housing is usually all the maintenance ever required.

                    REAR GRIP LOCK WASHER

QUESTION:  Every WW2 TSMG I have taken apart has a lock washer under the rear grip bolt but I can’t find a reference to that part on any drawing, manual or parts list. What is the story on that?

ANSWER:  You are right and I don’t have an answer for you. I have always felt that the 7/16" diameter grip bolt head was too small to be properly supported by the grip wood. I think they should have used ˝" diameter heads like those on the buttstock. When the Model of 1919 was redesigned to include a buttstock, Colt (or someone) had the smarts to use a larger bolt head. Very likely, the lock washers were field installed out of standard military hardware stock to act as a spacer to prevent the bolt from going too far into the trigger housing parts cavity as it was retightened.