Summary

This rubber band powered mini catapult is only about 10cm long, but it packs a punch! I used a super-strong rubber band that allowed me to launch items up to 1.5 cubic centimeters about ten feet across the room. The catapult's power is dependent on the strength of your rubber band and how far you pull it back.

Be sure to look at these accessories by alex80it

http://www.thingiverse.com/thing:865607
http://www.thingiverse.com/thing:865653

Instructions

I would recommend printing both files at 200 micron layer height and with at least 20% infill. No supports are needed if your printer can bridge a 1 cm gap. Most printers can easily do this.
Once printed, turn the lever assembly on it's side and slide it through the base. Snap the pegs of the lever into the base and test to make sure the lever moves up and down.
Slide a rubber band of about 5cm in diameter when relaxed over the spoon of the lever. In the lever there is a small cutout. Push the rubber band into this cutout. From there, pull the rubber band over the tower of the base, allowing it to rest on the small bridge. Continue pulling it to the base, pushing one side into each of the cutouts in the base. Pull the rubber band so that it rests behind the tower. See the picture for a visual.
The catapult is a little fragile, so be careful. Never aim at people or animals, even as a joke. Print and play at your own risk.

Mini Catapult