This version (of the very, very many potato cannons on Instructables) is helpful if you're UK based and have to work around the crappy stocks you find in our hardware stores, which, unlike American hardware stores, seem to assume you're an idiot who isn't interested in doing anything more complex than painting a skirting board. I bought all the components from Wickes. Homebase and B&Q were useless.
Full instructions are here: http://baddadgreatkid.blogspot.co.uk/2011/03/how-to-build-your-own-potato-cannon.html
Bad Dad website: http://baddadgreatkid.blogspot.co.uk