Backyard Ballistics by William Gurstelle. Great collection of home made ballistic devices for kids. Great projects especially if you happen to live away from the city and are friendly with the local fire department.
Build a Catapult in Your Backyard Click on that link and take a look at an enlarged picture of this book. Now THOSE are the guys I want to learn how to make a catapult from! Those guys don't mess around...