This is a semi automatic Lego rubber band gun that can also be easily changed to auto.. but then with rubber band guns semi automatic is more of a challenge to build- here's how I built my trigger mechanism... this is my remake of it that contains step by step pictures i hoped could be comprehended.. contact me with questions of comments please! =)
see it in action here : http://www.youtube.com/v/-8ly4yEf0Io&hl

Step 1: The Main Ideology Behind the Mechanism

ok so basically the way this works is mostly all int he gears- I have it geared so that whenever a rubber band is released (only one) the "trigger wheel" (grey beam in the middle of the sideways v) spins only once. so in other words one rubber band=one trigger wheel spin. Then the "trigger" (purple bent beam) is connected to a sort of ratchet mechanism that when I pull the trigger back it makes the ratchet release the trigger wheel which the rubber bands put enough force on to release one. Also I have a rubber band connected to the trigger (the trigger is just connected directly to the "ratchet"or "v" (because it wouldn't fit on the inside of the trigger frame because the two gears that hold the bands are there). soryr this isn't more detailed but if you ahve any questions feel free to ask.

Step 2: Have Fun!

I think loading is pretty self explanatory to any one with any experience whatsoever with rubber band guns; but what you do to load it is stretch the rubber band from the end of the barrel to the gear then you can pull the trigger in order to push the gear back too and I've had success with loading as many as 12. here's a video of me shooting it in my closet =P.