We checked out potato cannons on the web and based the fundamental design on what we learned there. We ended up going with ABS plastic pipes and associated fittings for the cannon. The combustion chamber has an inside diameter of 3 inches and is about 24 inches long. The cannon barrel is 1-1/2 inches by 36 inches. A reduction coupling connects the two pipes together. The other end of the combustion chamber is fitted with a female coupler and a removable (threaded) clean-out plug. Hair spray seems to be the propellent of choice so that's what we experimented with first.
The YouTube video, below, shows the cannon in action and also presents some of the construction details. The Instructable step by step instructions below are quite detailed to help the builder have a greater chance of success.
A second video giving more construction information can be viewed here: http://youtu.be/T7r6I8M4AG4
Be aware that this kind of device can be very dangerous
- it would be smart to read and view plenty of background material before construction and testing.