This contraption is made to be worn under a loose sleeve, hidden from sight. It shoots the ink cartridge of a ball-point pen. The trigger (a clothespin) is rigged to fire when I flick my wrist back.
**Update**
The instructable is now up! You can find it here.




























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Here's a good example:
http://www.indymogul.com/backyardfx/episode/BFX_20090119/quick-draw-sleeve-gun-build
Personally, though, I have a certain admiration for the more archaic weapons.
The end that is held closed by the spring is what holds the rubber band back, so the strength of the clothespin directly affects the tension you can have in the band. The rubber band is looped through a special catch that fits into the clothespin, so the rubber band is not directly pinched by it.
I hope that helps until I can post the whole thing.