Quoted:
That one was called [i]"Arena[/i]", and it was (very loosely) based on a kick-ass short story by Frederic Brown of the same name. No bamboo cannon involved.
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No bamboo cannon in the episode or the short story??
If you meant the episode, then I think you're thinking of a different one -- there was an episode where some first-contact "higher beings" pitted Kirk against a green lizard, the winner's species to be destroyed (although they weren't told this). Kirk created a crude gun from bamboo, black powder (sulfur/charcoal/saltpeter), and a diamond, shot the alien, but refused to kill it -- so the "higher beings" decided that Kirk's species was advanced enough to show mercy and might evolve into something worth talking to in a few million years.
Sheesh. I *am* a total geek.
Edited to add: I only watched it for the miniskirts. [:D]