We made these potato cannons early last year with the goal of being more compact than any of our previous models. They worked great for firing water balloons.
This is our most recent cannon - Cameron, Nick and I worked on it. We used this calculator to see how small we could make the reservoir and still get the distance we wanted. We made the barrel extra long and would cut it down if it was shooting farther than we needed (we wanted to hit the opposing dorm). As it turns out, they were overpowered (they can shoot things clear over the other dorm), but that is cool with us. We decided to leave the barrels at that length because they weren't all that big.
The barrel was 2" PVC, the tank was 4" and we used electronic sprinkler valves from home depot for the valve. To fill the tank we found a 1" to 1/4" pipe thread PVC joint that was perfect for connecting a quick-disconnect air hose connector to.
It fires water balloons! Cameron and Josue found that if you fill a regular sized balloon with water so that it is just small enough to easily fall down the barrel, it will fire and not explode in the air. We don't know how much it hurts to get hit with one of the flying water balloons and we don't want to find out. (It couldn't be much worse than getting hit by a water balloon from the other dorm in the eye, ya, it hurt for a while...)
The main problems we've found with this design:
- Not many, the design seems pretty solid so far. I wanted to put the valve on the reservoir side of the angle so that the cannon would have an overall smaller length, but Cameron and Nick wanted better 'heft'
I must add - PVC cannons can, and do, explode - so be safe and use the appropriate precautions. You are taking a major risk if you decide to build one.