The principle is simple, the rubber bands are notched onto the stepped back of the gun, but sandwiched between the two sides of the stepped back is another stepped piece, which is at a half step offset to the rubber band notches. Every time you pull back on the trigger this offset step pushed the bands off of their notches, allowed them to raise by half a notch height, then when you release they are resting in the next notch up and roll up to the top of it. At the top notch the pull action causes the band to fire off. And so each band simple 'walks' up the notches on the back of the gun until their turn to fire.
See the video here for me firing the completed gun: http://m.youtube.com/#/watch?feature=youtube_gdata_player&v=LWEdEw6hJv4&desktop_uri=%2Fwatch%3Fv%3DLWEdEw6hJv4%26feature%3Dyoutube_gdata_player≷=GB
The complication comes, once again, in the realities of having 8 rubber bands applying force to the structure of the gun. With 8 bands in place there is a significant torque on the back stepped sections of the gun. This twists them out of their half-step alignment with the trigger piece. So for the moment I have to apply a little force to counter the effect for the first few shots.
The other thing I find is that the force of the bands alone helps the trigger back in, but only so far, and not enough to quite release it's hold on the bands. I have to manually apply a little forward force to bring it all the way forward, so I intend to add a return band similar to the one on the modotchao to help the trigger always return all the way forwards. UPDATE: now with return bands
Another thing I found was that I put the handle a little far to the back, and had to make some adjustments to allow my hand enough room to actually fit. its not too bad but if I made another I'll aim to mount the handle much close to the centre of the gun 'barrel'.
This had been a fun build, and I like having it at work on my desk. A colleague has set up a little shooting gallery of lego men, and when I'm frustrated with a piece of code its good to be able to load up the gun and take a few pot shots :-)
In general I think the common thing I learned from both builds is that you really need to consider the effect of that much potential energy being held in check by the structure of the gun. There are a lot of forces even in just 8 rubber bands, and that can play havok with a nice design