The MK 1 Underground Submachinegun
Below is a series of thumbnails scanned from an "underground" publication of the 1970's. The design is thought to date to the early 1970's. The "comic book" style of print is simple and straightforward. While none of us have seen one of these guns in action, we have been told that the design is sound. The source did note that nowhere in the instructions was there mention of heat treating the parts and that the sear on the gun he built quickly wore out. He recommended heat treating the internal parts of the trigger assembly and the bolt if the gun is to be used for more than a couple hundred shots.
After looking at the design, it seems fairly simple, but it would still require a reasonably well equipped shop to produce the weapon and it would involve far more work than simply rebuilding a gun from a parts kit. There is no commercial source of MK1 parts. In spite of the "legal warnings" found in the print, US federal law specifically forbids construction of a receiver, but laws against construction of the other parts are not nearly as clear. I also noticed that the gun seems to be set up for a proprietary magazine. If such a gun were to be built, the builder might find it easier to adapt the design to magazines that can be obtained outright as opposed to constructed as I was told that the magazine is the weak link in the design. Most likely this design would be adaptable to Sten magazines but the Sten magazines would be modified to be compatible with the MK 1 magazine catch which appears to be loosely derived from the AK.
It appears that some skill with machine tools such as a metalworking lathe and milling machine will be necessary. Nothing high tech, but there is no way around the necessity for these basic tools. The smaller low tech machines of this type would probably be sufficient. Welding and forming sheet metal are involved in the design and it looks like the grip area is just a shaped loop of sheet metal. This could definitely be improved upon. The gun builder would probably be spending some time making the tooling necessary for forming the sheet metal components of this weapon. The maker will also need to heat treat at least some of the parts if the gun is to last more than a few hundred rounds.
The thumbnails are a series of prints used in the basic design of the gun. The design appears to be complete, but since there is no open source of parts available on the market, I think a builder could take liberties with the design and make some improvements, like a Sten compatible magwell, a better pistol grip, a folding or detachable stock and maybe a fore grip (but I think you are meant to hold the magazine). A silencer would also be highly recommended. I think one could be integrally designed into a lengthened receiver tube.