Well by the sounds of it we might have one of the only catapult designs. I can't speak for it's accuracy as it is not assembled yet, but we had 90% accuracy on the prototype made out of spare conduit and bike parts
.
The latest picture we have (rather outdated - now robot is painted black and has wheels on) be seen at:
http://marsbot.org/gallery/?g2_itemId=3069
Our inventor renderings (being completed as we build the bot
):
http://marsbot.org/files/images/bot_iso.gif
Hope you think it's decent! We will post a video of the speed of the throwing arm soon but let me assure you its impressive!