Here is a cool device that looks like a regular pen but can fire projectiles with the power of compressed air. The body is cut from a tube of carbon fiber using a fine-toothed hacksaw with the ends squared off and cleaned up on a lathe. The front cap is machined from brass. The rear end cap is machined from Delrin and has a Delrin piston inside.
The cap to allow firing is made from brass and aluminum. It allows the schrader valve stem to be depressed easily to fire the device. The above video shows the pen gun in action.