****UPDATE: This thing has a very short range. If you build this, you would be lucky to get 150 feet. I have since taken it apart and rebuilt it with a 24 inch barrel.
Cost to make - roughly $15 plus cost of Mad Bull XM PB4 unit.
My cost to make - $3. I had almost everything I needed already.
HOW IT WORKS:
The Mad Bull XM PB4 is a Rerservoir and Valve system for pressurized volatile liquid. For airsoft, this liquid is called Green Gas. When the Valve of the PB4 is triggered (opened) all the liquid flows out of the reservoir into the Chamber behind the Nerf Football. The liquid - being very volatile - QUICKLY vaporizes into a gas state. The volume of the Gas is hundreds of times Larger than the volume of the liquid state. This Rapid Expansion from liquid to gas is what Pushes the Nerf Football out of the launcher. For further explanation, please talk to your local high school science teacher.
UPDATE: I took this launcher apart and built a longer launcher. The new launcher is 24 inches long. It is a compromise between the pistol and the LAW which extends to 30 inches long. It abandoned this design because the range sucked.