http://www.youtube.com/user/joergsprave?feature=results_main
This guy makes every sort of slingshot you can imagine and puts them through their paces. If you want to know what is possible in the area of rubber powered projectile weapons, his videos can show you a lot. Be sure to go back to his earliest videos, he describes the different types of rubbers you can use and their pros and cons.
He also has his own website and forum, any serious questions, ask there. http://theslingshotforum.forumotion.com/
Ever the Tom Boy.....BTTT. :)