This is Suomi-konepistooli, the weapon that gave finnish soldiers strenght to push back the soviet forces during the winter war and continuation war. I used about seven hours to make this, and now I haven't got a single useful black piece left.
That huge drum magazine could hold about 70 rounds of 9mm parabellum carteridges.
The weapon was originally designed by finnish gun designer Aimo Johannes Lahti.
Here you can see the cocking handle. A rubberband holds it back so it will snap forward when you release it.