Sten
The Sten was England's main submachine gun during World War II, and was designed specifically to be easy to construct. It can be made largely out of stamped metal, and some versions took only five man-hours to construct. It was not exactly a fully-featured gun, but it was fully-automatic, and is certainly the best submachine gun in its price range that we are aware of. It fired 9mm rounds, and used German MP40 magazines so that ammunition could be picked up on the battlefield. They could be stripped down nicely, so their parts were easily concealable in bulk when disguised as parts for something else (such as car parts, pipes, or hydraulic cylinders). If you want to be all illegal and stuff, there are receiver plans for them on the internets. We're not posting them, because we're not that illegal.
This is kinda like a diagram, but shitty.


Here's the Sten with some accessories




The Sten was simplified even more in a Polish resistance model called the Blyskawica. Blyskawica means "lightning" in Polish, which is probably why they took the time to engrave little lightning marks on the stocks when there was a war on.
Here's a diagram.

Here's some eurotrash guy messing with his gun.