Need advice : Building a Catapult

[Conradus]

A friend of mine built a catapult about 10 years ago, when he was just in high school.
It looked fairly much like a trebuchet, but in stead of counterweight he just used a elastic (like the one you use to bind things on your bike) to shoot. So you pulled back the arm, put an acorn of whatever in it and released. It managed 10 m for sure.

[Tanit]

Could always build a Gatrophetes, that could fit the size requirement and still be life sized. And it was a siege weapon.