According to Wikipedia "a trebuchet works by using the mechanical advantage principle of leverage to propel a stone or other projectile much farther and more accurately than a catapult, which swings off the ground. The sling and the arm swing up to the vertical position, where, usually assisted by a hook, one end of the sling releases, propelling the projectile towards the target with great force."
My trebuchet is a bit different than some of the others that you see, in that uses a rubber band instead of a counterweight. I opted for rubber bands because they gave me better results with this particular trebuchet than weights and they are much more compact. This kind of siege engine is called a spring trebuchet, a form of a torsion trebuchet.
Here are some links, in case you want to get some more background info before you start:
NOTE: The measurements in the steps ahead assume you are using an Altoids tin, or any other container of the same size.
GO TO STEP 12 IF YOU WANT TO SEE A VIDEO OF
THE CURIOUSLY STRONG TREBUCHET IN ACTION
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Step 1: Parts and Tools
X2 -#10-24 tee nut
X3 -#6-32 round head slotted screw
X3 -#6-32 machine screw nut
X2 -#10-24 rod coupling nut
X1 -#10-24 threaded metal rod (should be about a foot long)
X1 -Altoids tin
X1 -Rectangular stick of good quality wood (I used poplar)
X1 -An assortment of sizes and strengths of rubber bands
X1 -Straightened coat hanger or other wire
X1 -Standard rubber aquarium tubing (make sure that it's a snug, but not tight fit over the machine screws)
-Cordless/electric drill and drill bits (a cordless drill will suffice for most of this project, but an electric drill really speeds up some processes)
-Hacksaw/bolt cutter (again, a hacksaw will suffice, but a bolt cutter will work better)
-An assortment of pliers