Here's the original easy-to-build design for a simple open-bolt homemade 9mm submachine gun. The interesting thing is that it could be made by an average person with nothing more than a hacksaw and drill - no machine tools or special skills required.
You need a username and password to download from this site, so use this one:
Click "Download" and select "Adobe Acrobat (.pdf)".
Here's some more gun designs from the same site. They are all good reads, although of course assembling most of the guns would be illegal according to the federal National Firearms Act. Make sure you download the .pdf, I have a hard time reading them otherwise. (you need the free Adobe Reader to view these files)
Another 9mm submachine gun how-to - this one is quite handsome and built from standard pipe fittings with minimal effort:
A .32/.380 caliber homemade SMG:
Homemade Semi-Automatic Pistol (.32/.380) (Appears to be much harder to make than the machine guns):
Home Workshop .22 Machine Pistol (looks like it requires machine tools):
Home Workshop 9mm Submachine Gun:
Home Workshop .50 caliber single shot Rifle (looks like it requires machine tools):
Home Workshop silencers:
Expedient Homemade Handgun Ammo and Reloading:
Expedient Homemade Shotgun Ammo and Reloading
Disclaimer: For educational purposes only. I have not tried to make any of these guns, also I'm not responsible if you make these guns and get hurt or get in trouble.