Hey! I've built three types of siege weapons for the Alliance, and I think I built all the ones mentioned in this thread so far, though I could be wrong.
First, I built the "huge slingshot" that Dan mentions. I believe that NH has that. It relies on surgical tubing pulling a sling forward from four sides.
Second, I built a large trebuchet for Alliance Caldaria (CT). You can see it in action here:
As for projectile, here we took some bouncy play ball, slit them open, stuffed them with newspaper, and taped them closed. the other projectile we made was the open cell foam pillows, about 1.5 feet in diameter (whatever the boulder rules allowed). The other thing being thrown is tons of packets, of course.
I beliveve, that although Caldaria owns the treb it is just sitting at my friend's parents house in NH. If anyone wants it and would pick it up they could talk to Caldaria and take it away, though I have to confirm it's still in NH.
Third, I built three identical ballistas and stands for HQ for the national event. They are stored there in PA, and you can go check them out any time or I can describe their details to you, including pictures which I took of the various parts, or we can chat and I can tell you all about them. They shot 3 foot pool large diameter (about 3.5", not the normal 2.5") pool noodles with embedded fins to spiral the projectile. They had an open cell foam tip and one D-cell battery embedded in the noodle, behind a closed cell barrier and the open cell tip. You could crank up the tension as much as you wanted, but the most we ever did was make a shot about 40-50 feet, with good accuracy and precision.
I'd be happy to discuss any of this with any of you interested. They were very fun to make, but the trebuchet proved a little unwieldy to use in actual combat, as it took a while to load, and could be easily avoided. Couldn't beat it in "corridor" battles though, and they add a ton of flavor. Even with the unwieldiness, I thought they were really a great time, especially if you make the projectiles worth a LOT of damage, like a Massive Eviscerate, or 50 Massive, or some such thing. Puts the fear of death in those PC's!
-Tim P.
Edit: Here's a video of a kid who made a "ballista" (it's really more of a slingshot), but it is extremely low budget and easy. It's really not a lot like the one I made, but maybe something like that is what you're looking for?