Here are some links that assisted us with our project:

The Spug Gun Technology Center, for high technology potato cannons .
http://www.goldmann.com/spudgun.html

Beginners Spud Guns, for those just beginning.
http://www.mshamash.com/spud/spudgun1.html

Historical Cannons, for old black powder type cannons.
http://www.nvo.com/cannons/

Tony's Spud Gun Page, an excellent assistance to any beginner.
http://www.tommasi.org/spud/

Pneumatic Potato Gun Archives, give useful information about air powered guns.
http://digitalexistence.net/spuds/

Potato Guns!!!, another page for building and using potato cannons.
http://www.geocities.com/unrealkrusty/