Here are the instructions and some tips for building a pen-gun. To get started you will only need a suitable pen and a rubber band.
Steps
Step 1. Use a pen that is suitable for the project
Not all of them can be transformed into a "rubber band gun". Find an inexpensive ballpoint pen with a "snap" mechanism (the ones with a button you press with your thumb) that will pull the tip out and retract. You should find a place where the body of the ballpoint pen unscrews, usually halfway, to gain access to its contents.
Inside you should find an ink cartridge (usually made of plastic with a metal tip), a couple of elements that make up the "button" of the snap mechanism and a small metal spring. When you are done, the pen will launch an adapted “bullet” constructed from the cartridge and a normal rubber band
Step 2. Unscrew the pen
Remove the ink cartridge. The spring may be attached to the plastic part of the cartridge; if so, do not detach it. If it isn't, look for it inside the body of the pen. If it was inserted on the writing tip of the pen, shake the latter to get it out or try to retrieve it with a pair of tweezers.
Step 3. If the spring is not attached to the cartridge, attach it yourself
Gently slide it over the tip of the pen along the small ridge most cartridges have. The spring will provide the gun with greater throwing power to the rubber band bullet.
Step 4. Insert the cartridge backwards into the section of the pen body provided with the button
It should remain well fixed thanks to the plastic housing that contains the snap mechanism. If inserted correctly, the tip of the cartridge should protrude from the body of the pen.
Before putting the ink cartridge, you should drop a small piece of crumpled paper inside the ballpoint pen. This secures the spring securely to the pen
Step 5. With adhesive tape, adhere one end of the elastic to the cartridge
Use transparent adhesive tape, insulating tape or resistant packaging tape and attach the elastic to the end of the cartridge sticking out of the ballpoint pen. Make sure you do a careful job, the elastic must not come off otherwise the mechanism could jam and you could hit yourself!
Step 6. Pull the elastic back
Find a suitable target (never a person or animal) and get ready to fire the bullet. Keep the elastic parallel to the body of the "gun".
Step 7. Let go of your "bullet" when you are ready to fire
The elastic should snap forward bringing the ink cartridge towards the target. Best of all, the bullet is recyclable. Retrieve the cartridge, aim and shoot again!
