This is a ratchet action style semi auto rubber band gun. It's designed for 6 bands but they tend to tangle on each other when packed that close. In my testing it works best with 3 or 4 shots. Started as a project for a side arm in the ongoing nerf war.
Demo Video: http://youtu.be/9XTkK8GNwKM
Print area on my setup isn't sufficient to print the barrel. Stronger/cheaper/easier to just dremel some slots into a wooden dowel then try and make it multi section. The two wooden rods need to be squeezing together a bit tight for the ratchet to work.
Print 1 of each piece. I would start with the slide and use that to cut the barrel slots while the rest is printing. In total you will need less than 2 feet of 5/16 rod.
Cut 6 slots into a 5/16 (8mm) wooden dowel spaced about the same as the teeth on the slide (pictured above). I used a carving bit in my dremel. Ideally the front side of the slots would be near vertical and the back side slightly sloped so the bands come off cleanly.
Cut the barrel to a reasonable length for your bands (~15 in) and assemble as shown. The band keeping the trigger in place should pull the trigger up so the ratchet works.
The barrel rod slides through the handle during assembly so it's easier to line up. Do a few test shots before you glue it in. You may also need to use a file to clean up the band slots or the trigger hole.
When loading, load from back to front. After you catch the band in the front and its slot make sure to pull it down firmly to make it less likely to tangle on other bands.