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 calls about this subject, I have apparently failed to make it clear and the last thing I want to see happen is someone getting themselves in trouble or orders the wrong receiver.  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 that they intend to finish it as a semi-auto using their SMG parts kit.  It's not going to happen!

There is hope.

Now that receiver production has been re-started, work on my "SA4" semi-auto Thompson gun design has also re-started .  SA-4 stands for 4th generation semi-auto. SA-4 uses a receiver that has external dimensions identical to the full-auto Thompson.  It also uses a frame that has the same external dimensions as the full-auto frame. It is impossible to tell the SA-4 semi-auto from the full auto from the outside other than by the markings. Of course, this means that standard Thompson box magazines are used and drums fit properly. However, the SA-4 is a different gun.  A machine gun bolt will not fit in a SA-4 receiver.  A machine gun sear will not fit in a SA-4 frame.  A machine gun frame will not fit on a SA-4 receiver. And if the SA-4 bolt is allowed to free cycle, it will jam against the next cartridge. The SA-4 is a semi-auto gun design. One of the best features of the design is that it incorporates a lever which looks like the Thompson rocker pivot. It is in the same place and is marked to duplicate the appearance of the pivot markings. What the new lever does is to provide a way to cause the bolt to lock open during drum magazine changes.

Actually, the design has been finished for some time.  The problem has been the bolt and frame.  The receiver is not a problem because it is simply a version of the Ultimax receivers I am already making.  The program changes for it have been made.  I could start making them any time.  I have partnered with a person to help me make the frames.  There is progress there but very slowly.  I could modify TSMG frames to work but that is not cost effective and would not produce the best results.  I have completed all the design and manufacturing engineering.  I just have to wait for help.  The bolt has been the real problem all along.  However, I have also partnered to make a variety of special Thompson bolts.  I have already done all the design work on the SA-4 bolt.  So now it is just a matter of getting the bolt project moving. The remaining parts needed are either simple to make or are variations of parts I used to make.  For those, I have all the special tooling.

My plan is to offer a semi-auto version of every "improved" and "special" model Ultimax receiver that I offer as described on the RECEIVERS & BARRELED RECEIVERS page of this website.  The semi-auto guns will be marked "RICHARDSON SEMI-AUTO GUN" and will be configured as pistols in that the frames will not have any provisions for a buttstock and horizontal forearms will be fitted.  That will allow me to fit 10-1/2" Thompson barrels to the receivers.  After a person finishes the receiver into a working gun, he can register it as a short barreled rifle (where allowed) and add the buttstock or leave it as a pistol.  This solves the problem of having to make special 16" barrels and having two configurations of guns to deal with.  If I got to that point I would offer two receivers: one with Thompson square threads to accept Thompson barrels and another with V-threads to accept special 16" barrels.

For the present, I will continue to push the SA-4 gun along with whatever help I can get.  Watch my BLOGS for updates.


       I found one unfinished Ultimax™ MS3 receiver in a machine after receiver production had stopped several years ago.  It will be finished and available soon.  Call me if you want this receiver. 


                                                                Shown below is the 2S3 and MS3 drawing.  




For those who want to build a semi-auto gun, I offer my 2S3 and MS3 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.


Bottom view of 2S3 receiver assembly showing internal parts fitted to an extended bolt travel receiver.

MS3 receiver assembly would appear the same.





   1) Spring, Sear Block, $5 

   2) Spring, Firing Pin,  $5   

   3) Pin, Firing Pin Retaining,  $3  

   4) Spring, Pawl,  $5

   5) Spring, Recoil (2 required per bolt),  $5 each

   7) Spring, Trigger, $5

   8) Hammer, $5

   9) Bolt Handle, M1 style,  $38

   12) Pivot Plate,  $38

   13) Pilot, Firing Pin,  $18


(These are temporarily out of stock.)
If it is desired to be able to use a 1921/8 style detachable buttstock on an N/K semi-auto gun, the frame (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 frame and drilling and tapping the frame. This requires a milling machine and machinist skills. (Malibu address shown has been changed to the Chatsworth address.)