I made some modifications to the original Seej penny catapult.

-the bottom of the arms was bumping into the table while loading the catapult, so I trimmed a bit of the arm below the bottom elastic bracket.

-I added an extra slot for the winding key and elastic further to the front of the catapult. This makes arms angle slightly steeper, which results in a slightly higer trajectory.

-Because i didnt have the right elastic, power was a bit low, to counter that I added some more elastic at the top of the catapult. To hold the elastic, I made an indent in the catapult arm and made the crossbar protrude a bit on both sides.

-Lastly, in case you grow tired of shooting pennies, I added a 'medieval' catapult arm. Shooting peanuts or perhaps a game of angry birds ( http://www.thingiverse.com/thing:11438 ) anyone?

The green catapult ha the original frame, with the modified penny arm. The red canon is fully modified.

That stack of 'amunition' are actually 10x10x10 calibration cubes. They come in handy now :)

For the original catapult only the stl files are provided. For the catapult arm, i hacked away at the original stl file in openscad. There was something wrong with the stl file for the side bracket, so i remodelled that. My openscad source files as well as zeng3's original stl files can be found in the zip file.