The Tar and Feather Gun* is for you. Live out all of your Bugsy Malone fantasies with this fun and messy contraption:
As far as I know, this has a fairly unique design among water (well, foam) guns in that it has a breech-loaded cartridge mechanism. The beauty of this is that you can quickly reload with a variety of ammunitions in pre-filled containers. More importantly, once you've fired the gun you can eject the depleted cartridge from the rear of the gun with an extremely satisfying *ka-chunk* sound.
Read on to find out how I built it!
There are quite a few things that can go wrong when you're building and playing with this gun. You'll be using sharp things to cut plastic. You'll be playing with drills, saws and toxic glues. You'll be putting a home-made pressure chamber under a lot of internal stress. And yes, you'll be shooting your friends in the face. Please be sensible. Wear safety goggles and use your tar and feather gun responsibly.
Whatever you do, don't leave the gun lying around loaded during an MIT frat party. Otherwise this will happen:
*a.k.a. "the personal pneumatic variable payload delivery device"