Geez. Of course, there's an exemption to the exemption. Pen guns are responsible for school shootings, terrorism, etc. And school shooting perpetrators and terrorists insist on only using legally-owned, NFA-registered weapons so if you deny them access to the NFA, they'll certainly not commit their heinous crimes.
Anyway, ok, no pen guns. But the other stuff is cool. And it's something you could build yourself.