Monday, February 21, 2011

Another project undertaken (very picture heavy)

For those of you who have been wondering about other projects I have in the works, here's a post for you.
I've taken a couple days off of working on the .50, mainly because I'm waiting for some steel to arrive with which I will work on the bipod and buttstock area.

A while back I procured a couple of Suomi m31 parts kits from a friend, one of which was 100% complete and the other was......not. I have enough parts to build two, but one of them is not going to have all the original stuff. I am saving it for later. First, I wanted to build one that would mimic the original M31. For those of you who are unaware of what an M31 is, go here.
I'm not too fond of the aesthetics of these guns, but the parts kits are cheap, and when it comes down to it, it's hard to argue with a 9mm carbine that holds 72 rounds of ammo in a drum. The biggest problem is, the doggone rifle is heavy. However, the parts kits are still available for around 100 dollars, and are fairly easy to machine the working parts for. One setback I've encountered is that it is originally an open-bolt submachine gun, and in order to be legal, none of the original full auto components can be used in their original configurations. This means converting the rifle to closed-bolt operation, which is a pain compared to the open bolt arrangement. I do not exaggerate when I say that a functional sub machine gun could be built in a matter of hours versus a semi-auto taking a handful of days. This is, of course, if you already have all the springs, magazine, bolt, and small parts.

To start, the receiver of this rifle has been cut to BATFE specs, which nowadays means it was torched apart into three different pieces. This pretty much leaves you with a pile of slag and unidentifiable metal scrap. I had to do a lot of research on this rifle in order to get some measurements that I could transfer to a new receiver. The new receiver could not be capable of accepting the original bolt, so after I took a bunch of measurements, I ordered some 4130 tubing that had close to the same outer dimension but a smaller inner dimension. This will allow me to turn the old bolt down on a lathe to fit snugly in the new receiver, but a standard bolt would not fit. The original bolt will also be highly modified so that it no longer works in an open bolt configuration.

I took some measurements and found that the receiver was 1.46" outer diameter and roughly 1.16" inner diameter. Since the magazine well components would have to be salvaged from what remained of the old receiver, I had to come pretty close to that outer dimension for the old mag well parts to fit snugly on the new receiver. There is also a section inside the original receiver that is about .75" inner diameter where the end of the bolt travels, right above the mag well. I was not about to bore out a 12" long piece of solid stock to two different inner dimensions. Luckily, I was able to purchase some 1.5" OD 4130 tubing from Aircraft Spuce and Specialty pretty cheaply. I ordered it with a 1.105" inner diameter, so that a factory bolt would not fit. I also ordered a piece of 1.5" solid round stock that I could machine to fit inside the receiver and weld in place that would take the place of the original .75" inner diameter section. This section includes the lugs that allow the quick-change barrel to be mounted. More on that later.

Here's the original kit and receiver in all it's......uh.....glory.

The new receiver tubing

 And, the old receiver

Some pics of the kit when "assembled"

 Now, I've sat on this project for a while, and finally this last weekend decided to get down to brass tacks and start on it. I began by taking some of the 4130 tubing and chucking it in the lathe and polishing it with steel wool. I then printed out a copy of the original receiver I had drawn in my cad program after finding a worthwhile blueprint of the receiver. I took this and glued it in place on the receiver and began cutting all the slots necessary. I left the material long and did not trim it to size until the machining was all done.

First, the magazine well

Next, the slot for the bolt/sear/hammer assembly on the bottom

A bad pic of the bottom of the receiver so far. The flash makes it look like it has been cut all the way through. It wasn't, the long slot on the bottom stops a couple inches shy of the mag well.

The ejection port. That funny looking knurled thing below the receiver is the thread-on end cap that holds the main spring in place.

Now, a slot on the top for the rear sight. Originally, the rear sight is what keeps the bolt aligned and keeps it from spinning in the receiver, as there is a flat running along the top of the bolt that meshes with a flat area on the bottom of the rear sight. The rear sight is also to be riveted in place.

Rear sight held in place
 Now, the receiver has the main areas that need machining taken care of. Now I can move on to trimming the receiver to the correct length. I didn't include any pics of the parting off procedure because it's a huge pain in the ass to part off metal to begin with, and holding a camera while dodging huge chips of metal sucks even more.

Now, once the receiver is trimmed to an overall length of 10.625", I turned the rear-most end down to an overall diameter of 1.46" to match the original receiver.

Next, I set the gear box on the lathe for 20 threads per inch, just like the original receiver has, and began threading. The reason you don't see the rest of the receiver is because it is inside the headstock of the lathe behind the chuck. That is one of the advantages of this newer lathe.

Threads rough cut, just a couple more passes at .001" each to clean them up,

And, the original receiver cap threads on snugly and cleanly.

Here I'm mocking up the receiver in the original wooden buttstock, with the mag well pieces I salvaged from the old receiver in place. A drum mag was used to get the spacing correct. I found out (almost too late) that if you set up the receiver for a stick magazine, the drum won't fit due to it being thicker than the stick mags. While the stick mags give you a healthy 36 rounds of 9mm on tap, what fun is it if you can't have a 72 round drum? So, a drum is used for mockup and tack welding.

Ignore the coat hangers. This is my garage, after all. I have to deal with having a dryer for a temporary workbench, and the significant other has to deal with having gun parts and tools all over the dryer.
 The receiver with end cap screwed on, drum in position, rear sight mocked up, and mag well tacked in place.

Some buzzing and zapping noises later, I had the mag well pieces MIG welded in place. Normally, these are riveted in place on the original receiver, however I see no need in riveting them on to the new receiver. No sense in making more work than is necessary.

That weird thing on the end of the receiver is the quick lock mechanism for the barrel housing. These have a quick release barrel assembly from the factory. I have to slightly modify the way this works so that an original barrel cannot be used on it to stay within the law. Reason why is because the original barrel is only 12 inches long. That constitutes a short-barreled rifle.
 Had to run uptown to get some parts from the local Horror Freight. Took Chloe the dog with me. Apparently, she loves car rides but gets bored from time to time.
 Anyway, the receiver is more-or-less done. I have to machine another part that slips inside the receiver right above the mag well so that the bolt and extractor will work in harmony with the barrel feed ramp, but before I do that, I have to modify the whole barrel assembly so that a short barrel will not fit on. Should I have a completed receiver sitting here that has the capability to accept a short barrel AND I have the short barrel sitting here (which I do), I would be breaking the law under the BATF's "constructive possession" law. So, I modify it so that a normal short barrel will not work, then finish the other part of the receiver.
I've mentioned before that in order to build some of this stuff, it takes me longer to do it in compliance with inane laws than it would otherwise. This last portion is an example of that. I have to do ____ before I can do _____ because otherwise I've built a/an machinegun/shortbarreled rifle/AOW/noncompliant light fixture/Improvised Explosive Device, etc.

Now, with that said, I do want a short-barreled rifle or two, I do want a AOW weapon, and I do want a machine gun. However, I will be going through the proper NFA channels to do so, and will pay my 200 dollar tax on each of them. It just sucks I have to do a bunch of mental gymnastics and odd preparation to ensure that at no point am I in violation of any silly laws.
Tomorrow I hope to work on the .50 BMG extractor, so it may be a few days before I pick back up on this Suomi, but I plan to have it finished pretty quickly.
More as I build it...............


  1. Dude, since you find the aesthetics on the Suomi to be wacky, try the KP 44. Think PPSh 43, but in 9x19.

    Parts kit I've seen runs around 70 bucks and up.

  2. I've seen those kits for as low as 50 bucks over at Centerfire systems.
    Yes, those are downright goofy looking, but I would still want to try building one sometime, just for kicks.
    I think I've seen prefab 80% receivers out there in the internet somewhere.

  3. I will buy one of your receivers!!!!


  4. Jacob,

    I'm not sure I understand your request. Are you asking to buy one of the demilled receivers that came with the parts kit, or are you offering to buy a completed receiver?
    Due to federal laws, I can't sell a 100% completed receiver since I do not have an FFL and I am not a licensed manufacturer.

    The closest one could come to that is to purchase a 80% receiver, similar to the ones sold at Hellbox armory

    Please let me know how I can help you build one of these great little subguns.

  5. I'd be happy with a 99% complete receiver =P. I understand that it would be legal for me to re-weld the original receiver, but do you think it would be legal for me to modify the original bolt to fire from the closed position by using parts from other weapons?

  6. Brando,

    I'd love to sell such a part, but when it comes to manufacturing a firearm, I have to limit it to being a 80% receiver, since I'm not an FFL.

    As far as your questions on this post, first off, let me say I'm not a lawyer. Do your homework and verify everything I tell you, don't take my word as the absolute truth.
    Now, you could use a SKS FCG, but that is going to keep you from reaching your parts count for the 922r requirement. You can make your own striker assembly or use a AR15 FCG or even a M1 carbine FCG, but no matter what, use something domestically made for keeping up with 922r.

    You might get away with using a long striker or moving the hammer behind the bolt, but I really don't think you'll have enough room in the receiver tube. A decent carbide end mill will cut through that insanely hard bolt in no time.

    Now, your receiver; you CAN NOT simply re-weld your receiver as-is. That would be re-activating a machine gun, and the BATFE frowns on this (actually, I suspect they love it when you do this, it gives them a reason to exist. Then, they get paid to kick in your door and arrest you.)
    If you want to re-weld the original receiver, you must weld in a blocking rail of some sort that will keep a full auto bolt from fitting in the receiver. Once you have the receiver modified, you need to remove the fixed firing pin and mill off the lower feed lips on the bolt,THEN mill it so the blocking rail fits the bolt so that it can never be used in a full auto gun.

    This might sound silly and excessive, but that's the nature of the beast. You can thank Fedgov. for that.
    Any questions?

  7. howdy i,ve read your project and have been doing research on this for about a week. i just procured a kp31 myself and would love to get a lil more info if possible. so far everything i,ve read has givin me new ideas and yours was veryy helpfull but like i said i,m waiting on mine to get here in the mail and would like to have someone a lil more knowledgeable to get info from if you dont mind my name is robert and my email

  8. sorry was a email sent sent to me.drkaries666 if so ididnt recieve it but like i said i should have my kp31 monday and need to ask a few questions regarding the reciever tube and see if you have a few either hand drawn or downloaded specs. to make sure that i have the right measurements for my projects and would also like to see if you know much on the semi auto bolts because i found some online and want to make sure i,m not wasting my money

  9. Yes Robert I emailed the address you listed above. Try emailing me this time,

    1. i have some similar questions can you help me out my email is thanks mike

    2. Mike, I emailed you as well.
      Let me known how/if I can help.

  10. I responded to your email. If you get it please respond so I know they're going through

  11. hey this is robert could you send me your email again or when we last spoke you and i dicussed a sg43 do you have specs on it or where i can find them also do you know how to make it semi auto not finding much on how seen a bunch already converted but cant find out how to convert it anyway any info would be great my email is drkaries666@yahoo thanks

  12. hey this is dustin i have a bunch of questions just for you and would like you to email me at

  13. would really like one of you 80% m31 receivers. just got mine in from HBA and it is a lot more work than claimed. how much is one of yours going to run?

  14. I am interested in one of your receivers. What would be the cost and could it be modified for a different caliber? Please email me with other tips and hints at what you have done is awesome and helpful thanks!

    1. I don't make receivers or any other gun parts for sale.

  15. I am in need of a blueprint I can glue to a 1.5" tube for milling the receiver. Do you have one? I have an 07 ffl but this is a personal project. Keep up the good work!