Semi-auto Suomi from scratch part I: the receiver: he's written about some easy projects lately: here's one that will be a real challenge. If you have the skill and patience, you can build an unusual semi-auto carbine for $200.This project started a while back when I did a Finnish Suomi M31 display gun (1/20/08). Since I appreciate well-built firearms designs, I decided I needed a functional Suomi for my military arms collection.
Federally registered full auto versions are very rare and priced well out of my income range. I knew of no manufacturers offering semi-auto rifle versions of this rather obscure firearm.
The only remaining option was to build a semiautomatic version to BATFE guidelines. I did some internet research and found there were several others building semi-auto Suomis. Some of these designs were very good, but I wanted one that was all my own.
So I set out to design and build a unique version. My design is certainly no better than others; it's just different.
My main design criteria were ease and economy of build. I deleted some original features to save buying parts. This would result in a project that looked pretty much like a Suomi M31, but it would not be an exact copy.
No semi-automatic carbine can exactly copy the original SMG, since BATFE insists on significant differences to prevent easy conversion to full-auto. The closer it gets to original configuration, the closer you get to a prohibited full-auto gun design.
The design featured in this article will be built to BATFE guidelines for building semi-auto firearms. It will also meet what is known as U.S. compliance issues.
As always, don't consider this story a complete how-to guide. Use it strictly as a starting point for your own ideas. This is a complex project that requires skill and experience; don't try it unless you have them.
I ordered a couple Suomi M31 parts sets from Cope's Distributing. These were priced at $59.95 and were in great condition. Suomi kits are one of the few remaining parts kit bargains on the ever-changing surplus parts market. I decided to use a U.S.-made fire control group, and ordered DPMS AR-15 fire control components from Brownells. I needed an AR-15 hammer, trigger, disconnector and their springs and pins, as well as an AR-15 firing pin.
I had used a highly modified M31 bolt and AR-15 fire control in a previous project for a SGN article, the SGN-9 (4/20 7/20/08), so I was confident I could use it in the Suomi project.
To start the design process, I laid out the demilled receiver sections and barrel in approximate original configuration and then laid out the bolt and AR-15 fire control group on top of it to see if there appeared to be room to fit it in.
This was the first "oh crap" situation. With the trigger in approximately the original location, I was about l/2"-3/4" short of having enough room to fit the fire control group behind the closed bolt. I studied this for a couple hours before the solution finally dawned on me.
I finally noticed that the rear portion behind the disconnector spring of the AR-15 trigger and disconnector did not have to be as long as stock, since the last 3/8" or so did nothing. If I cut off the unneeded portion I would gain 3/8" - 1/2".
I then decided that I could shorten the rear of the bolt about a quarter-inch to gain some more space. When the shortened bolt was closed, it was still a little too far rearward, so I decided simply to move the barrel about 1/4" forward. Once all these items were repositioned, there was just enough room to fit in an AR-15 fire control group in a new self-made semi-auto receiver.
While the Suomi appears from the outside to be of "tube gun" construction, it is not. The round tube part is just the top of the exposed portion of the receiver.
A lower portion that houses the fire control group is actually machined along with the upper portion. To ease the construction process, I decided to use conventional tube-type construction for the upper receiver and fabricate a new trigger housing that would attach to the trigger guard which in turn would be attached to the new tubular receiver.
The rear stock tang assembly and the rear of the magazine housing would secure the receiver in the stock, and the trigger guard with the new trigger housing would attach between those points.
Once I had the basic layout decided, I looked at the barrel setup. The original Suomi barrel was only about 12.5 inches long, and so would not meet the BATFE mandated minimum length of 16 inches. An extension would need to be fabricated and permanently attached to the barrel to meet or exceed the 16" regulation.
The original Suomi design also featured an easily removable/quick change barrel system. This is fine on a factory-built gun but is unnecessarily complicated to duplicate on a home build semi-auto project. I therefore decided to go with a simple barrel bushing into which the barrel would be inserted and cross-pinned as is common on many firearms designs.
This bushing would be plug-welded permanently in place once the proper position was determined. The permanently installed 16-inch barrel would also help alleviate any BATFE concerns about easily installing a prohibited short machine gun barrel on the completed project.
At this point, I had a basic idea of how I would build the project. The hard part would be actually making and locating the parts correctly for proper operation. There was going to be extensive amounts of parts fabrication and hand fitting of those parts since this was pretty much a "from the ground up" project, there were no plans for this endeavor.
If you choose to do this project be warned that virtually every single part will need to be hand-fitted. This is not a kit gun assembly project. It is gun making with your own self-made parts, combined with some factory made parts. This should not discourage potential builders, hand making and fitting parts is all part of advanced hobby gunsmithing.
One of the appealing aspects of this project was going to be cost. The Suomi parts sets were inexpensive. The AR-15 fire control group is widely available and relatively inexpensive. A few dollars worth of gunsmith quality springs will be needed. A couple pieces of 4130 tubing will be needed and aren't all that expensive.
Even counting miscellaneous supplies such as finishing supplies, screws and small pieces of steel, the cost should be under $200. The largest expenditure on this project will be labor. If one doesn't mind trading labor for cost savings like I do, then this may be your kind of project.
Demilled Receiver Parts Salvage
For this project, only a few of the Suomi M31 parts from the parts set will be used. Several items need to be salvaged off of the demilled receiver sections so they can be used to construct the new semi-auto receiver. You will need to remove the stock tang assembly and the front and rear magazine housing portions.
One question people unfamiliar with the M31 Suomi may ask, is where are the sides of the magazine housing? There aren't any! The magazine on the M31 was supported only by the front and the rear of the housing. Ribs formed on the magazine body interfaced with grooves in the front and rear portions to secure the magazine in place. This method is unusual but it does work.
Remove the tangs and magazine housing parts by cutting them out of the old receiver with a saw or grinder. Cut out the portion of receiver tube they are a part of. The round portion of receiver tube can be removed after the parts are separated from the demilled receiver sections.
The old receiver portion needs to be carefully removed so that the salvaged parts will fit the new receiver just as they did the old one. This is important to maintain the correct height of the salvaged parts. Use files or grinders to remove the old receiver tube sections. Do not over-grind or -cut since you need to maintain the same contour as the originals.
The "tube" portion of the new semi-auto receiver will be made from a section of 4130 chrome-moly seamless tubing that is available for steel suppliers and aircraft parts suppliers. I used 1.5"x. 187" wall tubing. Depending on size tolerances at manufacture, you will have an internal diameter of 1.125 to 1.130 inches. A piece 12 3/8 inches long was used for the receiver tube.
I chose this size tubing for two reasons. First, the external size was approximately correct but the second and most important issue was the internal size. Under BATFE regulations, you cannot make a semi-auto Suomi receiver that will accept the original diameter full-auto bolt.
There is an exception to this rule but we aren't using that construction method here. In our case the semi-auto receiver must only accept a smaller diameter semi-auto bolt specifically made for this semi-auto project.
The 1.5" tube is slightly larger than the original external size of the Suomi receiver. The original size was 1.460". The tube can be used full-size if you choose but some dimensions and fits will need to be modified to fit the slightly larger size. Since I had a lathe, I turned the tube down to 1.460". I also squared up the ends.
The bolt and buffer assembly of an M31 are retained in the receiver by an end cap that threads onto the rear of the receiver. The tube must be threaded to fit the cap.
The threads are standard "V" type threads and set at 20 threads per inch. The threads must be cut on a lathe, since there are no dies available for the odd 1.460x20 TPI thread size. Lathe cutting threads is very easy if you know how to do it but is extremely unforgiving on mistakes. One little mistake can ruin a threading job, so proceed with caution. Cut your threads and use the end cap as a guide for the correct fit.
To complete the receiver, the salvaged portions need to be attached and grooves and cutouts need to be made. The order to do these operations can be varied to suit the builder, but this is the order I did them in.
The first thing I did was to attach the tang section to the new receiver tube. To allow room for fire control group installation the front side of this tang was cut so that all that remained was the pocket to which the trigger guard will attach. The hole that previously allowed the charging handle to pass though the tang was welded shut.
The top radius of the tang assembly was shaped so that the tang was flush against the receiver when it was installed in the stock. The front to back position was determined by leaving enough clearance for the end cap to be installed later.
The tube and tang were clamped together, then removed and MIG welded. Square fit is important here, so be sure the parts don't shift during removal or welding. I placed narrow beads along all sides and then ground off the excess so that they wouldn't interfere with stock fit.
The M31 receiver is held in the stock by a combination of the rear tangs and the front of the stock interlocking with the rear of the magazine housing. This housing also contains the magazine catch assembly. The receiver and tang assembly were fitted in the stock and the rear of the magazine housing was fitted to the front of the stock.
The top of the housing must be contoured to fit the receiver tube. It also must be sized so that it will be in the same up and down position as originally on the old receiver so that the magazine will be located at the correct height when retained by the magazine catch. It must fit squarely in all directions.
I made spot welds at the top edges of the housing to lock it in place before removing the receiver and finishing the welding. The back sides of the housing were then welded to the receiver tube. Use weld techniques here that will limit heat distortion. Grind the welds smooth with the receiver so they don't interfere with stock fit.
Before the front of the magazine housing was welded in place, I cut out the magazine opening in the receiver tube. I used the previously installed rear housing to align a magazine with the receiver and then drew a line around the magazine on the receiver to mark a rough location for the opening.
I then cut well inside this line. I then used a magazine as a guide to finish the opening to size. I cut the opening very close to size so that the side would help support the magazines. This opening needs to be sized for the types of magazine you will be using.
The shape of this opening is slightly different for drum and stick magazines. Since I am not a big fan of drum magazines, I cut the opening to accept stick magazines only, but you may decide to fit it for both types.
Once the opening is sized correctly, the front of the housing can be installed. On the original Suomi M31, the rear half of the housing was part of the forward receiver. The front half contained a groove that allowed the barrel shroud to slip between the housing and receiver.
Since I eliminated the removable barrel feature, this housing needs to be contoured along the full length of the top to fit the new receiver. Once shaped it can be MIG welded to the receiver tube along its sides and front. Use a magazine installed between the front and back housings to align the front section. Be sure the housing is square and that the magazine is square in the receiver before welding.
Clamp it in place, making sure to allow enough clearance to allow for easy magazine removal. Use narrow welds here to limit heat distortion of the receiver. Once they're welded, smooth up the welds for appearances.
With all the parts welded on the tube, the remaining cutouts can be made. A clearance slot needs to be cut in the bottom of the receiver to allow the hammer to access the bolt. This 7/16" wide by 2 1/4" long slot was cut right in front of the tang.
The original M31 featured an unusual method for operating the bolt. A handle that extended out of the rear of the tang section internally engaged the bolt. It looked like a little bolt-action rifle handle. Since this would interfere with our centrally mounted AR-15 hammer, this feature was eliminated.
This modification also removed one regulated foreign part from the project. For this reason, and to ease the build, I went with a more conventional bolt handle that would extend through the left side of the receiver. A 5/16" wide "starter slot" was cut in the side of the receiver.
This was to allow for opening the slot to full size later to maintain precise bolt alignment. This was another area where I made a small mistake and cut the slot way longer than needed. It should start about 3 3/4 inches from the rear of the receiver and extend about 4 inches forward. This slot size can be increased later to allow for full bolt travel.
An ejection port needs to be cut in the right side of the receiver. This port must be centrally aligned with the extractor on the correctly positioned bolt. This cut out was also cut in a "starter size" well under finish size to allow for final fitting later.
The finished cut out will be about l 5/8 inches long and about 5/8" wide. This cut should begin even with the front edge of the rear magazine housing. I located the starter cut at about the 1:30 position around the receiver tube. Once the bolt is fitted, the ejection port can be opened up on which ever side is needed to maintain correct alignment with the bolt and extractor.
In Part 2 (4/20 issue), Matthews will go to work on the barrel.