Technically, you probably could, but it would be hard. I've come up with some designs for one in a lego gun, but I haven't actually made it yet (the idea works for knex too though). Basically, you make a gear (snowflake piece and small green rods on each spoke), and then you make another gear with a trigger attached to that one though. Then, design the shape of your gun, and you want to put the single gear at the firing area where the regular trigger would go, and put the gear with the trigger attached below it, so that they mesh, and when you pull the trigger, the top gear needs to be COMPLETELY RELEASED (not just turned) from the other gear with the trigger attached. (for this concept, see my C3 crossbow). Then, what you need to do is attach on the side of the single gear, one of those orange straight pieces, and then somewhere else on the gun (either to the orange piece's right or left), put another orange piece, so that they mesh together (and the one can spin freely, while the other is attached to the gear which is meshed). Then, put some rubberbands on the single orange piece, so that everytime it is moved it snaps back, and makes it harder to rotate. Then, load all of your rbs on the firing gear, and make a magazine, and attach it to the gun. When you let go of the trigger, what will happen is the gear will start to rotate, but will be slowed down by the rotation of the 2 orange pieces, which will allow the rbs to come off slow enough to fire correctly (they need to come out fairly slowly because the magazine cannot reload the next shot very quickly). Probably the fastest you will be able to get with a fully automatic gun (except if its a minigun), is about 2 shots/second.