Here is the classic hunting with cannons web site:
http://www.buckstix.com/
Click on the Mt. Howitzer tab for the story of building the cannon and the "How to Hunt Wisconsin Whitetail Deer with a 12 pound Mountain Howitzer Cannon" story.
Click on the Coehorn Mortar tab for building information and "How to Hunt Feral Cats in Wisconsin with a 12 pound Coehorn Mortar Cannon" story.