This uses a really advanced trigger mech. It fires 3 rounds of oodammo from the back of the stock. The trigger must be reset by hand after firing. Build from the pictures. I don't have the entire gun made with real knex, but I did build the trigger to ensure that it works. It has a 3 round magazine and fires the bullets consecutively as the trigger is pulled down (you can fire full or semi auto this way). Attach 3 elastics (one for each round in your guns max capacity) to each of the blocks (6 elastics in all) at the front tip of the barrel.
If you build this, please take picture(s) and/or make a video. Ask me first if you would like to make an instructable for it.
PICTURES UPDATED APRIL 26th, 2010
The second picture shows the new front end and new elastic placement.