you could just use a blowgun, and release it quickly. it has the same effect.

oh, my bad. i was talking about propane and butane, the only fuel i ever use in any cannon.

diaphragm opens faster, but pistons dump more air. taht's why supah valves are so loud and so popular: they have the fast opening of a diaphragm, and the air dumpage (hehe) of a piston.

yeah, just toss in the pen, empty pill bottle, screws and ignitor. none of those except possibly the ignitor will arouse suspicion, but even then, i don't think that they will give you any crap about that either. as long as you dont take any flammable gasses with you...

since this is somewhat on topic, i made an ultra mini piston the other day.
it has a 1" tee with 3/4 coupling as the piston, but it took a long time to grind down the coupling for it to fit in the tee properly.

how thick are the walls on those extiguishers? i had thought about this before, but never wanted to waste the money on buying one just to be disappointed with the results. if they are thick enough to tap, i smell a new project coming. Oh, and what all is in that foam?

i always thought that 1 1/2" pvc sch 40 fit them perfectly, but i could be wrong... have you tested it cole?