With nothing more than cotton swabs, glue and a length of music wire, some very effective darts can be created. They may be a little small for the 6mm barrel at first, but a little roughing up of the cotton with a fingernail is all that is required to make them expand to the barrel size. In about 5 minutes you can have dozens of darts ready to fly.
Remember never to shoot these at any living thing. These darts can easily do damage at more than 50 feet. The cardboard cutouts shown in the video make excellent targets!
Once again, here is a link showing how to make the mini cannon itself: How to Make a Mini Cannon from a BBQ Lighter