I can not believe the number of people who have a total lack of understanding about making a semi-auto Thompson.  Since I am getting endless calls about this subject, I have apparently failed to make this subject clear and the last thing I want to see happen is someone getting themselves in trouble or ordering the wrong receiver.  So this new section has been added to concentrate all semi-auto postings on this website here.  The most important thing to keep in mind is that a machine gun receiver can not be made into a semi-auto.  If you want to make a semi-auto, you must start with a semi-auto receiver.  And a semi-auto receiver is not interchangeable with a SMG receiver and will not fit a SMG parts kit.  A semi-auto is a different gun.  So far I have had at least 10 people order a machine gun receiver and then make an off hand comment to me that they intend to finish it as a semi-auto using their SMG parts kit.  It's not going to happen.


Shown below is the 2S, MS drawing.  It can also be used for the 2MM & M2 bolt cavities and the back end of the 2M2 receiver.




For those who want to build a semi-auto gun, I offer my 2S and MS (“Shop™”) receivers which rely on the Numrich/Kahr (N/K) gun design approval. (There is no approved design that allows a TSMG receiver to be made into a semi-auto.) These receivers are 1/10” less high than a TSMG receiver and are interchangeable with the corresponding N/K 1928 and M1 style semi-auto receivers except that the firing pin clearance groove in the top of the N/K trigger housing must be extended rearward. This is because my 2S and MS receivers are meant to have a bolt cavity that extends all the way to the back of the receiver like a TSMG. This has nothing to do with the gun being semi-auto or full auto. This is done in order to use the extra bolt travel to absorb the recoil rather than depend on ultra stiff recoil springs to do it. This requires the recoil springs holes in the bolt to be deepened to enable the use of longer pilots which are necessary to retain the recoil springs. These receivers require a modified N/K Bolt, modified N/K Firing Pin, special Recoil Springs (2), special Recoil Springs Pilots & Plate, special Firing Pin Spring, standard M1 TSMG Pilot or my M1 TSMG Sling Pilot (used in place of the 27A1 Firing Pin Spring Pilot), special Hammer and my 2S or MS bolt handle. The 2S and MS receivers also differ from their N/K counterparts in that they are made from the same heat treated steel alloy used in my TSMG receivers, the rear sight is properly positioned and the quality is much better.

If it is desired to fit a Thompson SMG barrel (10-1/2” long) to the semi-auto and not register the gun as a “short barreled rifle”, it is possible to make a pistol by using a horizontal forearm and a trigger housing that has no provision for a buttstock if the receiver can not accept a standard trigger housing. For that application, I offer trigger housing rails on the receiver that are wider than normal so that a standard trigger housing can not fit onto the receiver. A standard N/K trigger housing can be used by deepening the receiver rail slots.

Bottom view of 2S receiver assembly showing internal parts fitted to an extended bolt travel receiver.

MS receiver assembly would appear the same.


My original version extended bolt travel receiver, which I call the 2S-1, used a drop-in 1928- style bolt handle in the standard N/K bolt handle hole. This required the bolt handle hole in the receiver to be placed farther rearward than it is on a 1928 TSMG. The parts illustration for that version is shown below:


To create a more authentic 1928 style gun, a new 1928-style bolt handle was designed to be mounted farther forward on the 27A1 bolt than is normal for the 27A1 gun. The new bolt handle combined with a bolt handle slot on the top of the receiver that is identical to a 1928 TSMG, creates a receiver assembly that virtually duplicates the appearance of a 1928 TSMG. I call this version the 2S-2. It is the one I am currently offering in my catalog. The parts illustration for that version is shown below:


The extended bolt travel MS configuration has not changed.




If it is desired to be able to use a 1921/8 style detachable buttstock on an N/K semi-auto gun, the trigger housing must be fitted with an adaptor. I manufacture an adaptor to do that. My Buttstock Adaptor Kit installation instructions are shown below. Installation of this kit requires milling a mounting flat on the trigger housing and drilling and tapping the trigger housing. This requires a milling machine and machinist skills.

Short Barrel Semi-Auto (31 Aug 08):

If you are planning to install a short barrel (legally) on your N/K gun, you will need to change the barrel, grip mount and forearm or foregrip to TSMG parts.  That adds up to several hundred dollars in cost and you still end up with an N/K receiver with its poor quality, super stiff recoil springs, oversized bolt handle and the rear sight in the wrong place.  If you are getting the parts from a TSMG parts kit, you end up with several more hundreds of dollars in unused kit parts.  I suggest that a better way to go is to order a barreled 2S or MS receiver from me, a trigger housing assembly from Kahr and just make a gun.  That requires a little more skill and the finishing of the receiver, but you end up saving money and ending up with something better.  If I ever get my trigger housings back in production, that would be the only way to go.



                                 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.


Semi-Autos (24 Apr 08):  For some reason I keep getting phone calls from people wanting to order a machine gun receiver which they intend to finish as a semi-auto.  I just don't have the time to go over this again and again.  I thought I had made it clear in my various postings that it is not possible to do that.  Assuming that the problem is that people aren't finding everything I have to say about this, I have established this new section on this website where everything I have to say about semi-autos will be located.  I hope this will be helpful.

Thompson Semi-Autos (26 Jan 08):

There seems to be a lot of confusion regarding semi-auto versions of Thompson guns. The yearning for a $20 drop-in semi-auto sear that will convert a Thompson into a legal semi-auto is understandable but not realistic. So, I will attempt here to try to explain the factors involved and the choices that are reasonably available.

Like it or not and regardless of the ingenious designs you may have for a semi-auto Thompson, we are all stuck with the Government’s design requirements. Of course, there are no fixed specifications set down in law that designers can work to, it is the interpretations and political agenda of bureaucrats that dictate a vague set of requirements that usually are summarized by some statement like “every case is different so we must examine the gun in order to make a judgment” or something like that. However, there does seem to be a pattern that has emerged that we can call the “requirements” and here it is as I understand it:

1) A new receiver must be used and if configured as a pistol, must never have had a buttstock style of trigger housing fitted. You can make a pistol into a rifle but you are not allowed to make a pistol out of a rifle.

2) The barrel must be 16" long if a buttstock can be fitted unless the gun is registered as a short barrelled rifle. If no buttstock can be fitted and the foregrip is not a vertical type, then it is a pistol and any length barrel can be used. However, if a person makes a pistol out of a parts kit and retains the short barrel, the Government may conclude that the person intended at some future time to install the short barrel and therefore, has an illegal gun.

3) The gun must fire from the closed bolt position.

4) The bolt face/firing pin arrangement must be such that if the sear is removed from its semi-automatic functioning of re-engaging the firing pin after every shot, the gun will jam on the next cartridge.

5) The receiver must have some means built in to it in order to prevent the operation of an original full-auto bolt if installed.

6) The trigger housing must not be able to accept a full-auto sear.

7) An unmodified full-auto trigger housing must not be able to fit on to the semi-auto receiver.

To date there are only two approved designs. The Numrich/Kahr (N/K) and the Polston. However, there are three choices:

A) N/K. The advantages of this design are that it is the lowest cost and readily available. The disadvantages are that they solved 5) above by reducing the height of the receiver by 1/10". This requires the use of modified box magazines and interferes with the fitting of a drum. Because the receiver height is not the same as a Thompson, the rear sight is not in the Thompson position, and Thompson parts are not used, the gun doesn’t look all that much like a Thompson. Also the quality of the gun is poor.

B) Polston:    Polston has gone to the extreme opposite of N/K. He insists on using virtually every part of the Thompson parts kit to obtain absolute authenticity even to using the lock mechanism.
The advantages are that his gun is a Thompson in appearance as well as configuration as is possible within the Government‘s limitations. The disadvantages are that the gun is complicated and expensive to make. His insistence that all the Thompson parts be used and extensively modified where required, makes his gun a virtually custom one off gun which does not lend itself to manufacturing. That is why he is having difficulty trying to find a manufacturer to “manufacture” it. A manufacturer would design the gun for production of say 500 guns with new made parts. By modifying the original parts instead of making new ones, the task is to modify one gun 500 times. He may decide to do just that so another disadvantage is that gun is not available any time soon.

C) Richardson: That’s me. I offer an enhanced version of the N/K receiver (both 1928 and M1 styles) that puts the rear sight where it is supposed to be and extends the bolt travel so as to reduce the effort needed to cock the gun. Also, the receiver is made to a much higher quality than N/K. Otherwise it relies on the N/K approved design. The only disadvantages compared to the N/K gun is that the buyer has to finish machining the bolt cavity, it is more expensive and there is a wait. (At one time I manufactured a special trigger housing for my receiver I called the SA Adaptor-Frame. It solved the magazine fitting problems of the N/K. It was also made 1/10" taller than the N/K so the overall dimensions of the combined receiver and trigger housing were the same as a Thompson and so the gun looked like a Thompson. The disadvantage of the A-F trigger housing was that it required all special internal parts. The real disadvantage is that I just don’t have the time to make them.) A Thompson trigger housing can be modified for use with my semi-auto receivers or N/K receivers but there really is no advantage now to doing that since Kahr now offers 1928 style trigger housings and it seems a shame to damage a Thompson trigger housing.

So, obviously, there is no one simple way to go. The easiest and cheapest way is to just buy the N/K gun and be happy and use your parts kit to make a display gun. Well, at least try to be happy but save your money for a real Thompson unless you see that ad for the $20 drop-in semi-auto sear.

Semi-Autos (9 Dec 07): There are essentially 3 semi-auto configurations suitable for the Thompson. One is a “Hammerless” design in which the firing pin is spring loaded. It is retained in the “cocked” position by a sear mechanism. This is the configuration used by Numrich/Kahr (N/K) and is common in hammerless semi-auto pistols.

A second configuration is the “Hammer” design in which a spring loaded swinging hammer is retained by a sear mechanism. When the hammer is released, it strikes the firing pin, driving it into the cartridge. This design is used in M16, AK47 and FN-FAL guns and is used in Polston’s design.

A third configuration is the “Split Bolt” design. In this design, the machine gun bolt is split into an upper portion which incorporates the bolt face and extractor and a lower portion which contains the firing pin. It operates similiar to the “Hammerless” configuration. This design is used by UZI and is the design I used in my semi-auto Thompson design.

There is no particular advantage of one design over the other although when trying to design a semi-auto version of the Thompson, the “Hammer” design is the most complicated. The problem is with the ATF approvals. When Numrich got their approval, the ATF was allowing a bolt with a full feed lip. When Polston got his approval, the ATF was insisting that the bottom portion of the feed lip be removed. That did not cause a problem with the UZI but did with the Thompson because of the position of the ejector on the Thompson.

Polston very cleverly solved the problem with a hinged feed lip (which he calls the “Rock & Lock”) for which he has applied for a patent. Because of his patent claim, it is unlikely that anyone will be able to get any design past the ATF which will not infringe on Polston’s invention. The N/K design is simple and cheap although poorly designed and made. The Polston design is complicated and more expensive but retains more Thompson authenticity especially since it retains the Blish Lock mechanism. My “Split Bolt” design is irrelevant because I will never have the time nor inclination to put it into production.

As I see things, the N/K design and the Polston design will both have a place. Those who want the most authentic semi-auto Thompson they can get will go for the Polston gun. The majority will settle for the N/K gun. With that being said, there are still the problems caused by a poorly designed and manufactured N/K gun and Polston being a ways away from being able to deliver any hardware. Polston’s website is

My plan is to support Polston’s efforts and try to improve the N/K design even more than I have. I have redesigned my new 192S and MS receivers to have a full TSMG bolt travel and use a standard M1/M1A1 Pilot. The 192S uses my special SA Bolt Handle, redesigned SA Pilots & Plate, special SA Firing Pin Spring Adaptor and modified N/K bolt. Because of the increased bolt travel, softer recoil springs can be used but now the N/K firing pin clearance slot in the trigger housing will have to be lengthened. If I can find the time to get my Adaptor-Frame SA trigger housings back into production, I will be able to offer a much improved design over the N/K trigger housing assembly by incorporating aspects of my “Split Bolt”gun design and offer a complete SA receiver/trigger housing kit. For the time being I will offer a complete receiver kit but to mount it on a standard N/K trigger housing will require the slot modification.