While reading an article in Guns and Ammo about the Sons of Guns shotgun gatling episode, I noticed a lot of negative comments about the gun they built. The original gatling concept was abandoned when their design exploded, but the resulting inclusion of 3 Saiga shotguns in a rotating mount seems to some people to be a copout.
Be that as it may, the resulting gun may not be very useful for anything, but that isn't the point. These guys are making something, and they are using their hands, brains and wits to make it. Most of us are just consumers, we watch what others have filmed, and use stuff that other people have made, but it is liberating to take the helm of some technology and steer it your own way.
Some caution is advised here, so don't run out and start cranking out machine guns. Homemade guns are pretty unforgiving on areas of your body that are behind them, like your face. There is plenty to learn before you start hammering out big guns. That said, all the information in the world is about free now, so start reading, get your grinders and files out and go to work.
There is a pretty good video on YouTube showing the Saiga gun in action, if you can stand the commercials...
Some other links...
The MultiMachine, open source machine tools
And a little background on how guns are made...