Introduction: Full/Semi Auto Oodammo Slingshot
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 (you should be able to use up to 5 rounds with a little less range per shot) and fires the bullets consecutively as the trigger is pulled down (you can fire full or semi auto this way). Add elastics to the trigger, 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.