Steve-
Although I have absolutely zero experience with punt guns, and I've never built a trebuchet, I share your interest -- and have been interested in everything from ray guns to heavy artillery since I could read -- and I started that at a very early age.
I have been involved in some pretty outrageous projects though, and for quite some time had a program on my computer that was essentially a trebuchet calculator which took into account swing, payload, counterweight, and track (possibly more variables, can't quite remember at this moment)
Ten or so years ago a group of friends and I were poised to build one but as it always works out, we ended up split off in separate projects instead.
One of those friends ended up being one of the support characters in the mythbusters show. I don't remember if she ended up building a trebuchet during her contribution to that series or not. I just note that to give you a sense of the kind of characters who were involved in that caper.
The machine would have ended up at the burningman event. We wanted to throw cars with it.
I think a punt gun would be easier to build than a trebuchet.
A good piece of drawn-over-mandrel (DOM) tubing of good enough alloy (you can just do the hoop stress math on whatever gauge/load/velocity you were after and choose an alloy, or just go 4140), a suitable breechplug, and some way to anchor the thing to a boat should be all you need -- that and a touch hole for ignition.
Go for it Steve!
--Tinker
