How to make a small pneumatic gun out of a pen. It can be pressurized over 100 psi and shoot small nails into wood. THIS IS DANGEROUS if you choose to make this I am in no way responsible if you injure your self shooting it or if it explodes while your pressurizing it.
*I didn't know this but someone has already done a better job of a pneumatic pen on spudfiles http://www.spudfiles.com/forums/pneumatic-pen-gun-project-t7858.html
thanks to Surfguy21 for this info
Step 1: Parts
For this project you will need a bic pen and another piece of tubing (I got mine from a mechanical pencil) that fits inside the front peice of the bic pen if you remove the ink cartrige. You will also need a Zebra mini mechanical pencil (got from staples) or an eraser that fits inside a bic pen and you can drill a hole through. You need a schrader valve (the ones used on bike tires), and a airsoft bb.
Step 2: Tools
All you need for this is a knife and some epoxy.
Step 3: Get the Schrader Valve
The best way to do this is to get an old bike innertube and cut the schrader valve out, then sand off about 3/8 of an inch of the rubber off exposing the brass.
Step 4: Taking Apart the Pen
This is really simple just disassemble the Bic pen and keep all the parts.
Step 5: Prepare the Eraser
All you need the Zebra pencil for is its eraser and spring. The spring is pretty self explanatory, just bust apart the pencil and take the spring out. The eraser will serve as a seal it will go on the end of the larger tube (not the ink cartridge) and be sealed when the airsoft BB touches it. In the drawings pink is the eraser.
Step 6: Prepare the End Cap
This step is a lot like the eraser step just cut off the end cap of the pen so you end up with a plastic tube.This is the black part in the middle of the drawing.
Step 7: Assemble the Valve
In this step you combine all the parts you have been preparing. Get the large tube insert it into the end piece of the Bic pen and epoxy it. Then place the eraser doughnut over the other end so that some of the eraser is sticking out past the plastic tube, then epoxy that in place.
Step 8: Make the Piston
This is very simple, just glue the airsoft BB onto the end of the Bic pen end cap(this may need to be sanded so it slides easily inside of the pen tube, it shouldn't be airtight so feel free to sand lots off) and then glue a spring inside of that. See pictures for detail.
Step 9: Make the Barrel
The barrel is just the ink tube that has been emptied of ink. Use some wire and a small piece of cotton to clear out the ink. Then withing two milimeters of the end of the ink tube poke a hole straight through the tube. This will allow air to enter the barrel once it pushes up the airsoft bb. See pictures.
Step 10: Put It Together
Start by epoxying the schrader valve into one end of the pen tube. Then insert the piston (pen end cap with airsoft bb and spring attached). Then put the valve piece in (the tube with an eraser and the front a pen), epoxy this in place. Then insert the barrel with the hole inside of the pen tube.
Step 11: How It Works/ Firing
Its pretty simple, the spring pushes the bb down onto the eraser, this seals the barrel off which is inside the tube. As you fill the schrader valve with air, all the empty space inside of the pen acts a chamber like in a potato gun. Then you push the barrel inwards, it pushes the bb up and exposes the two holes in the barrel this provides a place for all the air pressure inside of the pen to release. You can shoot pretty much anything that fits down the barrel, I found that cutting the heads off nails works pretty good. I normally pump it up to about 100 psi, this is easy because of how small the chamber is. BE CAREFUL even though it is small it can get sharp objects up to high speed and could easily put your or anyone's eye out.