There's a fella on YouTube showing off some serious MacGuyver skills. He's made a bullpup-style machine gun that fires BBs, using a 20-ounce soda bottle and some over-the-counter parts totally $15. He shows it off firing at a target, and it looks like it does some serious damage.
It's kind of crazy that YouTuber Nighthawkinlight can build such an effective projectile weapon with naught but a plastic bottle, a brake line for a car, an air compressor and blow gun component, but the video is proof. It's loaded up with Airsoft BBs, though we still wouldn't want to be on the business end of this thing without a little protection.
See the bottle gun in action in the video below.