I made this trebuchet a while back, and found that it could hurl a small fishing weight about 100 feet each time. A trebuchet can fling a projectile by using the counterweight to pull up the arm and the sling. The trebuchet was built to throw their projectiles high and not as far as possible. In medieval times the trebuchet would be used as a seige weapon to throw corpses over the walls of a castle or fort, causing the inhabitants of the castle or fort to contract diseases and to die or weaken.
Here is a link to the original plans http://www.schenectady.k12.ny.us/users/pattersont/IBDT%20Website/FinalActivityMay05.html