I dismantled this gun a while ago, but I've decided to set up a crude explanation to show how it worked, for anyone interested in Lego rubber band guns. When I say it's crude, I mean it! This is not the images of the real mechanism, just a mimic built in 5 minutes. But my gun is not good enough to get its own step-by-step instruction, so I figure this is quite the most I can do.
A couple of main things to remind you: the trigger slides on a rail, not swing a round a hole like other Lego RBGs. I was too lazy to build a rail, so I just move it on the surface of my desk. C'mon, this is just to BRIEFLY explain how it works! For novice (real novice), this gun let rubberbands "shoot themselves": it holds bands on a tooth gear, somehow prevents the gear from rotating until you pull the trigger. Pull the trigger, the gear can rotate and let a band, or bands fly away. For more info, click here
Now, back to this gun.
So, this is the semi-auto mode, as you can see, the "hand" that is supposed to be powered by rubberbands by a geartrain is stopped by a piece at the lower left corner. When you pull the trigger, the whole thing slides back, letting the "hand" go, releasing a rubberband. Then, the piece on the upper right holds the "hand" back. Only until you release the trigger could the "hand" complete one rotation.
Things are different in full-auto. The piece on the upper right corner is turned away, so nothing is in the "hand"'s way when you pull the trigger.
This is the locked mode. The upper right piece is now blocking the "hand"'s way with its longer side, so the "hand" can't no matter the trigger is pulled or not.
This looks simple, but the tough part is how to how the mode selector in the right position, apply enough force so the upper right corner mode-change piece won't be moved by the force of the moving "hand" in semi and locked mode, etc. And those stuffs are real pain! But I strongly suggest you build a 3-modes RBG, if you are already a Lego RBG expert or if you want a dramatic beginning on Mocpages.
All kinds of questions and constructive comments are welcomed. I will improve this mech and build a better gun soon!