Try some spear gun rubbers - think bungee cord. Tune it by varying the
stretch. Or think of all the old weapons like crossbows, catapults,
trebochets etc. Also look up spud guns, there are some that are powerd
by just air with some sorta positive feedback diaphram valve and then
there are the ones driven by fuel/air explosions useing something like
hairspray for fuel.
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Looking forward,
Al Shinn

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