The pneumatic pen gun using a coaxial piston valve is fairly well known, while the exhaust valve design is nothing new I lay claim to having made the progenitor back in 2007 for a spudfiles.com forum contest as featured here.
The method of filling and firing is shown in this simple animation.
Since my first effort I have gained access to machine tools and have made several much improved versions. Here I will show the steps in the manufacture of a pen gun with a carbon fiber body.
The main body is cut from a length of 12mm OD - 10mm ID carbon fiber tubing roughly with a fine toothed hacksaw, and the ends are then squared off and cleaned up on a lathe. In order to protect the finish, masking tape is wrapped around the tube before placing it in the lathe chuck.