Here is a great modification of the Ogre catapult. It is lighter and stronger. It gets much better performance. My thanks to Mark and Thomas for making this upgraded version and for sending me the pics and information. Below is what they had to say about what they did. This modified catapult throws a record breaking 26 meters which is over 85 feet!

My name is Mark, and my friend Thomas and I needed to build a catapult that weighed less than 2 kg and would throw an egg more than 8 meters for our science class.  Last year we built a version of your huge Wyvern torsion catapult and it was a smashing success.
 Unfortunately that catapult was much much much too heavy to work again this year, so we had to start from a new design, and that design was your Backyard Ogre.  To add more power however, we made a couple alterations, and to share these is the reason I'm emailing you today.  For ours, we made all of the components from 1" square bars of varying lengths instead of your 2 by 3's.  Pressure treated lumber was strong and light enough to hold together at this smaller size.

 We also added support struts out of wood instead of inset wood, for simplicitys sake.  For the power, we took your original ideas and expanded 4 fold.  We used 4 18" bungees attached to two different eye holes on both sides of the swing arm.  Other adaptations include a different hinge mechanism, which will be pictured, a simpler bungee securing system, a PVC friction reducer, and a 39" swing arm.  With these changes, our catapult threw the egg a record breaking 26 meters!  

When I took it home and was throwing full water balloons, I was able to eke out 60 feet maximum throws.  I will attach multiple scaled pictures that you could post to help others replicate our results.  If anything remains unclear, please email me back, and I would be happy to explain anything. Thanks for all your help!

The Backyard Ogre - 3 foot Band powered catapult

Here is the full tutorial on how to make the Ogre catapult.

The Big Book of Catapult and Trebuchet Plans

This book is a compilation of step-by-step instructions for how to build nine different types of catapults and trebuchets, including plans for: A PVC Pipe Trebuchet, A Tabletop Trebuchet, The World Famous Floating Arm Trebuchet, The Historical WarWolf Trebuchet, A Rat-Trap Catapult, The Mangonel (or Onager), The Greek Ballista, a Petraria Arcatinus And the Scorpion II. Each set of instructions include dimensional drawings of all wooden parts, lists of required hardware and sources, an abundance of photos, diagrams and detailed descriptions of the assembly process. Additional sections include historical notes for the different types of machines, warnings and safety suggestions, tuning tips, notes on scaling, tools and techniques and additional notes on even more kinds of catapults.