A good catapult design?

Question

in shop I'm planning on building a catapult its gonna be about 40 inches long and about 20-26 inches wide and I'm thinking of using springs to pull the middle board down and send the object flying i want it to launch at least an 8lb shot put a good distance maybe 50 to 100 yards would this work???? what should i use as springs??? I'm thinking those rubber bungee cord things or some like springs because they stack real well

would this work or should i do something else

ps i dont want a ballista

Answer 1

A speargun is a balista based on rubber cord.  

Adding rollers at the front could make the use of double lengths of rubber cord possible, wich gives a more constant pull.  26 inches is very wide and unstable,  the slider rail should be narrower.  An open rail that let the slider jump out freely only held back by a rubber cord, is probably more robust.

The alternative is the catapult

See http://www.answers.com/topic/catapult for some last century improvised examples