This is a handheld potato cannon. It is also the first project I attempted to build. The curved back tank serves to increase tank size while remaining out of the way. An unintential feature of this is that the tank can serve as a hook, which makes it very convenient to hook into a pocket for easy transportation.
A minor problem encountered was that I wanted the valve to be on the handle portion of the cannon, which meant that between the valve and barrel there would have to be a 45º turn. To prevent any potato from getting stuck in this elbow, I put a stopper in it consisting of a hole drilled through the pipe with a nail driven through it and with both ends of the nail filed off. The result is that there is a piece of metal in the back of the barrel which effectively does its intended job.
This cannon has also shown a remarkable ability to fire .5" paintballs. For added performance, A certain paintball storage tube was found to fit snugly inside the barrel, and paintballs snugly inside the tube. Thus, the paintballs become a much better fit and fire faster and more accurately than you might expect.
I also managed to make a royal mess with the PVC cement...