Volume 10 of Professor Parabellum's Practical Scrap Metal Small Arms.
This volume provides plans and instructions for one of the simplest Professor Parabellum designs, a 100% DIY 9mm SMG made from mostly square or rectangular metal sections. This makes it particularly amenable to DIY with simple hand tools.
"The 9mm submachine gun design described here is extremely basic and can be put together using very limited tools and materials. The design was intended not only to be easily constructed, but also easily remembered in terms of measurements and relationship between the (very few) components. The trigger mechanism for example is simply a steel rectangle and a square! The design makes use of standard sizes of square steel box section, which is sleeved together to form most internal components. No milling machine or lathe is required.
In all its a basic 'entry' DIY design yet entirely field-able. Its compact trigger housing makes it lightweight and with the magazine-well naturally resting against the users left wrist it's particularly wieldable. This makes it perfectly suited if you ever find yourself in a trench or decide on a spot of emu hunting from inside a truck cab. The demonstration model accepts standard 9mm STEN magazines, though plans for a homemade magazine are also provided."