https://www.youtube.com/watch?v=n1wV3lmbSv4
Now, it is single-shot - but because they are so easy and cheap to make, someone who wanted to cause mayhem could just make and preload several dozen, and discard them as they shoot them. Still plenty deadly.
That's the thing, really... guns aren't particularly complicated mechanically to begin with, and they are even less complicated if you strip them down to bare essentials - which, if you only intend to use it at a close range, still leaves them quite functional.
On the other hand, the reason why guns are so simple is because most of the complexity is in the ammo. While that can be reloaded, it requires primers - and those can't easily be made at home.
So, if you want gun control that actually works (in a sense of preventing dangerous people from owning an overly destructive device), it has to be primarily about ammo, not guns.