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.
Step 1: Main 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.
Step 2: Front Endcap
A front endcap is machined from brass. The part which enters the tube has some shallow grooves cut in order to allow better adhesion of the epoxy what will be used to fix it in place. In this case, a 1/8" BSP male thread is also cut in the front in order to allow the fitting of attachments later.
Step 3: Barrel and Valve
A length of 5.5mm ID brass tubing is cut for the barrel. This is epoxied the the front endcap. On the other end of the barrel, a 8.5mm diameter Delrin sleeve is added in order to increase the area in contact with the piston seal and reduce wear.
An automotive schrader valve is stripped of its rubber sleeve and trimmed in preparation to be mounted in the rear endcap.
Step 4: Rear Endcap and Piston
A rear endcap is machined from Delrin. Inside, a Delrin piston is fitted, with an outer diameter closely matching the inner diameter of the endcap bore. The piston has a small 3mm rubber disk attached to its front end with a small M3 bolt.
Step 5: Body Assembly
The front endcap is epoxied into one end of the carbon fiber tube. The rear endcap is epoxied in the opposite end, and the schrader valve is epoxied into the endcap. Once the epoxy has cured, it is ready for test firing in this configuration.
Step 6: Firing Cap
A cap to allow firing is turned from brass and aluminum. This allows the schrader valve stem to be depressed comfortably in order to fire the device.
The charging, loading and firing sequence is shown in this video:
A bicycle shock pump is used to bring the device up to pressure.
Step 7: Subcaliber Insert
An insert which threads on the front endcap allows the caliber of the barrel to be reduced to 4.5mm in order to fire smaller projectiles.
Step 8: Other Variations
Various different configurations are possible.
In small calibers such as the 3mm example shown in the videos below, penetration can be quite impressive when high pressures are used: