Wednesday, February 23, 2011

Suomi M31 update

Well, the metal for the .50 BMG still has not arrived, so I did a little more work on the M31.

As previously mentioned, I have to do this work in a certain order to prevent becoming a felon. I certainly don't want to blog about something and toss up pics that would later lead to my conviction, should I have a brain fart and forget to do something by the book. With that in mind, I had to modify the barrel first and make sure it was legal length before I made a modified receiver extension that the barrel mounts to. Had I made the receiver extension first, I would have then had to modify the barrel to fit this extension, and by the definition of the ignorant stupid asinine ridiculous law, I would have then broken the law in terms of constructive possession, as I would have had the components on hand to assemble a short-barreled rifle. So, the barrel gets modified and lengthened first, then the receiver extension gets made. And when the receiver extension is made, at no point is it capable of accepting a factory short barrel.

Am I being paranoid? Probably. But, being paranoid and cautious usually keeps one from being a guest of the state in a concrete room.

With that said, I figured the easiest way to make the receiver extension assembly unable to ever take a factory 12" barrel would be to turn down the end of my barrel and thread it, then thread the receiver extension to accept the barrel. This would be turned to 3/4"x16 t.p.i.
Here I chucked up the barrel and started to turn down the chamber end down to .75" OD from its original  diameter of .8"

 Next, I just happened to have a 3/4"x 16 tpi tap, so I figured that was good enough. So, I set the lathe to 16 threads per inch and cut the threads about .04" deep.
 Next, I had to make sure the barrel was at least 16" long, so I turned down the last 1 1/2" of the barrel to 5/8" OD, being very careful not to bung up the crowning.

With the barrel threaded on the chamber end and turned down on the muzzle end, I took a quick picture or two.

  Next, since the factory barrel is 12" long, and I was going to slip a piece of steel over the barrel and silver solder it in to make it legal length, I turned down a 5 1/2" piece of 1" diameter hot roll steel to 3/4" OD and bored it out to 1/2" all the way through and then bored it out to 5/8" ID on one end about 1.5" deep. This would allow it to slip over the factory barrel and act as an acceptable barrel extension, according to the ATF.

The whole assembly is yet to be silver soldered together, but that will probably happen tomorrow. More pics then, perhaps in a couple days.


  1. That ain't paranoia it's just good sense. I like the new project. The barrel addition is a nice solution.
    I'm thinking about buying a Sterling SMG. It's a finished gun because I do not have your skills but a nice piece anyway.

  2. I like those Sterlings. Classy looking guns, at least more than a Sten, very utilitarian looking.
    You actually going to plunk down the serious money for a real Sterling SMG, or are you referring to a semi-auto Sterling rebuild?
    Thanks for the compliment on the barrel. I haven't had a chance to do much more as I've been fighting the flu.

  3. The Mad Ogre, through his gun company, is offering a complete semi auto. The price is actually reasonable enough for me to seriously consider one. If I can convince Lu that is. I think they bought the parts kits and are building complete guns.

  4. Wow. Just looked 'em up.
    Looks like I may have to find me a sterling kit :)

  5. But man, does that thing need to be SBR'd and possibly suppressed.

  6. SBR and a suppressor would be sweet!