This is something that they don't show you how to do when you buy the sets. Using less than 15 pieces (and a rubber band), this can shoot its ammo a good 4 feet, depending on how strong the rubber band is. It can also be modified to shoot farther, higher, faster, and more.
Step 1: Get pieces
This should not be very hard, especially if you have a lot like me. Colors may vary.
Step 2: Assemble body...
Put them together like this, but the spacing is only to show the pieces better.
Step 3: Assembling...
Snap on the other side like so...
Step 4: Make firing pin.
Put together this part with the rubber band strung in the eye of the pin. Then put it through the holes on the base.
Step 5: Streatch rubber band.
Loop the ends of the rubber band like so. It should not be too loose, but don't pull it to its full potential. Adjust as needed.
Step 6: Make ammo
The ammunition is just 2 pieces put together to make a ball shape (sort of).
Step 7: Shooting the cannon.
Pull back the pin until the trigger flips down in front of it. Load the ammo on top of the two prong-like things in the front. Fire by pulling the trigger to the side. Dry fires ( or too much power) can result in a launched pin and hurt fingers.