I decided to go with a long barrel since it helps the accuracy of the gun quite a bit and also adds a little power. For the barrel, I chose a 4 foot long PVC pipe with a diameter of two inches. The combustion chamber was a 1 foot long PVC pipe with a four inch diameter. Also needed was a conversion piece to connect the barrel to the combustion chamber and a screw-on cap for the end of the combustion chamber. The parts can easily be purchased for under $20 at a store such as Menards, Home Depot, or any other home improvement store.
The construction was pretty simple; PVC epoxy was used to permanently affix the chamber and the barrel via the conversion piece. After letting it dry for a day, the gun was ready for its final component: the ignition device. Since this was my first, I selected a lantern sparker (uses flint and steel) and drilled a hole in the end cap that was large enough to hold the sparker. I also used extra superglue to make sure the sparker did not fall out.
My first test used a potato as the projectile and White Rain hairspray as the combustion fuel. I first jammed the potato to the lowest depths of the barrel and then sprayed a healthy portion of the hairspray into the combustion chamber, sealing it with the end cap quickly. The potato in the barrel prevented the hairspray from escaping in that direction. Finally, I took aim at a target in my back yard, cringed, and twisted the lantern sparker. BOOM! The potato flew out, making a funny sound, and exploding into chunks as it struck the target.
I used the potato gun many times and had a lot of fun. Besides being messy, the potatoes generated a truly foul smell as they rotted in the yard. Unfortunately, the lantern sparker broke after a while and I haven't bothered to fix it. When I do get around to it, I plan to use an electrical ignition system, which is much more reliable and powerful if implemented properly. Also, electrical ignition can be safely conducted from a distance if you are testing fuels that could possibly explode.
I was pretty conservative with my combustion fuel throughout testing, although a friend of mine filled his with propane. When he fired it, the joint between the combustion chamber and barrel failed, sending the barrel flying down the street into the neighbor's yard. This illustrates the dangers of homemade firearms! If you decide to make your own, don't use highly explosive fuels and treat it like a normal firearm by not pointing it at anyone or looking down the barrel.
Relatively Safe Potato Gun Fuels:
It is very hazardous to use gasoline, propane, ethanol, gun powder, and many other highly combustible fuels!
Potato Gun Ammunition:
Bags of Flour
Rocks or Metal Bearings (Use some sort of wadding to keep them from falling into the combustion chamber)
The legality of potato guns is questionable. They are considered homemade munitions and many states will allow you to be charged in that regard if some mishap occurs. One fellow's friend was accidentally killed and he was charged with a criminal offense. In any case, if you decide to build one, don't be a moron with it and go around vandalizing property or shooting living things. Potato guns are dangerous weapons and can easily kill people, therefore they should be treated like any other firearm.