@Barto I've never heard of such things.
Oh, I haven't either. Just seems pointless to launch a non-flaming projectile.
If you're not already up on it (which seems unlikely), you might look into pumpkin chunking. There are two classes, pneumatic like what you seem to be building, and mechanical, which is what always interested me. In the modern era people have really improved trebuchet design quite a bit. Between floating arm, floating axle and murlin trebuchets they're really getting some insane power. Something to be said for being able to launch a 36" CRT TV into the neighboring suburb.