How to improve our catapult..

So our (school) engineering team was assigned a task to construct a catapult, one that would NOT make any permanent changes to the base at all. We cannot use engines or explosives. Which left us with a Torque Catapult. It requires a trigger system that can be fired 3 meters away from the catapult. While everything works, we aren't getting the distance we want. We have to launch a Riffle ball.

Pictures of it: http://img705.imageshack.us/img705/1782/catapult1.jpg
http://img24.imageshack.us/img24/3969/catapult2.jpg

The main power is being supplied by surgical rubber, however we plan on attaching two bungee cords along with the surgical rubber. The trigger system we currently have is very simple, a string tied around the catapult arm, and then around a hook. We tie a string to that string, and when pulled, it DE-attaches from the hook, launching. We just don't get the distance we need when fired, and I am hoping for any suggestions/ideas.

I would suggest strong bands around the base of the part of the catapult that emits the projectile. I would say the band should be at least 1-2 inches of an inch thick, depending on how big the catapult is. That way the catapult with make more of an impact on the projectile which will eventually pick up enough momentum to travel at terminal velocity.

If you stay with this set up you need to look at the angle of the arm at release.

Also you could consider adding attributes of a trebuchete

Your arm might be to long or have to much flex in it

On a tension catapult look at the models of ones used in the past the arm was not much longer than the frame