Building a semi-auto M31 Suomi

Hello Again,

I wanted to start this thread as a way to give a little back to the forum that has tought me so much. I cannot help people with gun knowledge as much as I would like on here because there is always an expert out there that is way more qualified, so I thought this might be helpful and maybe even fun.

A few months ago I decided I wanted to build a semi-auto M31 Suomi in 9mm from a torched parts kit. I have been very busy and I have hardly had time to work on it until lately, and I still dont have time for too much. The first step for me was research, research and more research. I joined another forum that specializes in gun building/modifying. I decided that I would make my design around a hammer fired system vs the popular striker fired system. I started by drawing up some prints for the reciever that I planned on making. I decided to make my own take on the TNW 3 peice design. I build the middle and rear components myself and bought the TNW front section from Apex because I was not set up with a rotary table that would be capable of cutting the lugs on the front portion. The middle section, I made using a TNW middle section as a guide. Its so close that I dont think that TNW could tell the difference. I used tool steel for the middle section because I was worried that hot rolled or cold rolled would not stand up because it is thin in some spots. I then made the rear section out of 1 3/4 hot rolled steel. I turned the OD down to 1.457" to make it match the OD of the middle section. I then drilled and reamed the ID to 1.195", though I did lose some accuracy at one end due to the reamer coming in and out so much. I believe I lost just .0055 on one end which I think is pretty good considering it is 8.5" long. The ID I choose was smaller than the original and I did this on purpose. I dis this to comply with the BATF. They want either a blocking bar or a reduced ID to ensure an original full auto bolt could not be easily installed. I will have to turn down the bolt to match this new ID of the reciever. I then cut a slot on the top measuring .400" for the rear sight and then drilled and tapped an 8-32 hole on each end to mount the sight. I then milled a slot on the bottom the measured .320" wide for the charger handle and for the hammer. I then threaded the rear end of the tube for the cap that closes off the end of the reciever. The thread was a M37x1.25 which was a bit of a puzzle on a standard lead screw lathe. Lastly, I made a couple of side plates for the trigger housing out of some .073" pate I had around. That is .010 smaller than the original side plates, but I will not matter in the end. The reciever is now finished except for welding. The parts are designed to slide together and then plug welded to complete it. For the trigger group, I designed and built a hammer fired system using some original parts and keeping the look of the original from the outside. I planned on using it but decided I would purchase one from TNW when I got my front reciever section. Even though I was confident in my design, I decided saftey was my main concern. I like the peice of mind of a proven system and its cheap insurance. I still have a long way to go but I feel like I am making progress. I still need to weld the reciever, extend the barrel, mill slots in the bolt for the hammer, design/install a firing pin in the bolt, reduce the diameter of the bolt, fit and fine tune the trigger, asemble and test the rifle and then apply a type of spray on finish. I plan on doing this over the next month or two and will slowly be updating as I can. I will try and photograph as much as possible and describe as much as I can. My hope is that it may help a future forum member to make one himself or at least think about it. I hope you enjoy and please feel free to ask questions or for clarification on anything.

mdrim13

I finally got around to working a bit more on the gun today. Between school, work and everything else its been a busy last couple months. I have been trying to get the fixed firing pin out of the bolt so I can fabricate a standard semi auto style firing pin set up. The fixed firing pin was a small pin that was pressed into a hole in the front of the bolt and then reinforced by cross pin. Removing it will be tough. This is very difficut because the bolt is so hard. It is classified on the Rockwell C scale as a 60 (very hard). I could not get the firing pin out. I first tried with a hammer and punch, then a 10 ton hydralic press an finally a 100 to hydralic press. The 100 ton press would have worked but I kept breaking the pressing bits as they are only 5/32 in diameter. I got tired of messing around with it and put it on the EDM (electric discharge machine) drilling machine today. For those who do not know, the EDM drilling machine is a machine that uses a copper wire with a carbon core to electricly burn a hole in metal, espically hard metals. It works by creating a very small controlled and precise electric arc similar to an welder that burns the metal away. These are incredibally accurate. I drilled out the center of the fixed firing pin and attemptedto drive out the remaining thin tube left by the EDM. It would still not budge. It will soak in oil overnight an I will try again tomorrow. If that does not work I will go to heat and then a carbine drill and lathe. Attached are before and after photos of the EDM drilling. The before picture is one off of the internet as I did not take one and it show the bolt from multipe angles for reference. The top one shows the original fixed firing pin.

mdrim13

Hello All,

I have not had much time to work on the build lately. Tomorrow I will be drilling out the rest of the original firing pin with an EDM driller. I will open up the hole slightly. The next step will be making a firing pin. I have contimplated many options for this and I have come to the decision of making one out of a greade 8 cap screw. I will turn down the threaded portion and make a point at the end for the firing pin itself. The spring will be secured by the head of the cap screw one one end and by a counter bore I will make from the back end on the other side. The pin will be held in place by a cross pin as many civillian firearms are. I will post pictures of my work as time allows.

mdrim3