A way to make a real k'nex fully automatic gun Answered
UPDATE: I have begun to construct a gun out of this. It doesn't shoot very far in the video because I am using a motor to power it, but once I complete it I'm hoping that it'll be pretty powerful.
Original Post: It occurs to me that you could use a crankshaft with a motor or wound up rubber-band to push and then subsequently pull a firing pin in your gun. This coupled with a hopper that can push rounds into the chamber, and you would have a truly fully automatic gun, as opposed to a ton of single-shots all on a chain (which is cumbersome to load).
I'm not good at all at actually designing k'nex guns (not anymore anyway), but I did put together and example of a crankshaft, and have attached photos and videos to this topic. The actual crankshaft at the back of the example could easily be elongated so that you could have your firing pin travel farther, and if you put that axle on a small gear attached to a larger gear which is on another axle with a rubber-band or motor to turn it, you could get at least 20 shots from the gun. Reloading it would also be really easy, as you could have a crank to turn the main axle allowing the rubber-band to build up potential energy once again.
Sorry if this is confusing, I'm not the best orator or typer as it were, but I think the pictures will be a lot easier to understand.