This is a very easy pen gun that doesn't require anything except the pen and a rubber band.
Also this is my first Instructable so please don't beat me with heavy sticks.
Step 1: Step One: Disassembling the Pen
This step is pretty easy. All you have to do is un-screw the cap and the little point... thing...
Step 2: Step Two: Assemble the Gun
Basically, you just put the rubber band around the pen and then twist it and put it around again 2 or 3 times.
Step 3: Step Three: Firing the Gun
Insert the pend cartridge into one end. Push it all the way through until it comes out the other end. Grab it and the rubber band together. Pull back the rubber band and let it go! It goes really far.
Note: Keep your thumb on the place where you wrapped the rubber band around. This will keep it from unraveling.