The pieces of plastic in the video in no way make up a firearm. This is a desktop toy, and is a single hole punch that can punch through 2 sheets of paper at a time. Think of this design as an elaborate hole punch.
Therefore, it shares the same trigger design, as well as similar sear design. I found the piece of filament as the spring to reset the sear a poor idea, so I changed the sear so a rubber band can be attached to assist in reset.
The current model weights in at 50.1 grams and costs roughly $0.75 to make.
This printed model pictured is the third version of the hole punch. It is composed of 5 pins, the bolt, the receiver, the trigger and sear.
This project is not yet completed. The bolt head is not long enough to punch paper, and the rubber band should not be wrapped around the gun, instead attached somewhere near the muzzle.
UPDATE: The 4th revision of the bolt fixes the shortness of the punch part. The trigger and sear have also been modified. The plan for the 4th revision of the receiver is a trigger guard. For this reason the trigger has been shortened. The sear has a small notch cut it in now for the rubber band which should cause less friction.
UPDATE 2: Bolt 5 prevents the first forward pin from being broken off. It also adds a second pin hole on top for larger rubber bands.
See assembly video here: http://www.youtube.com/watch?v=b6bk4KfecXE