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If you've never heard of this hobby/sport/activity/whatever you've been missing out on a lot so listen up good:
A basic spudgun is a small pipe joined to a larger one with an endcap on the free end of the larger one. Basically, the idea is to shove something down the small pipe, called the barrel, and put an explosive mixture in the larger pipe, close the endcap and set it off the mixture. The result is the expanding gases in the larger pipe push out the thing you shoved down the barrel, rather quickly. Obviously one would need something to set off the explosive mixture. Usually, one would use a piezoelectric sparker from a barbeque to make a spark jump across two screws that are stuck in the side of the chamber (the larger pipe). Some people use a flint sparker used in Coleman stoves instead. These are more expensive so they aren't as common.
The most common projectile fired out of a spudgun is, as the name implies, a potato because they are heavy enough, cheap and when they are shoved down the barrel, they can be cut at the same time to fit the barrel exactly. you can put all sorts of stuff down the barrel including things that light from the flame of the combustion as they come out.
The potato gun described above is what we call a combustion gun, since it uses combustion to propel the potato. The other kind of gun is a pneumatic gun, which basically consists of a tank of pressurized air, a barrel and a valve. Something is shoved down the barrel, the valve is thrown open and the pressurized air propels the projectile off and away. These types of guns have many advantages and disadvantages.
Disadvantages that come to mind are that one needs an air compressor to recharge the gun after each shot. If you use a small compressor that runs off say a car battery, it takes a long time to recharge the tank, and it's loud. If you use a large compressor, you need access to a plug and it's heavy and hard to carry around. Compressors are also expensive if you don't already have one. If you use a bicycle pump then it's lots of work and takes a long time. Another problem is the valve, it has to open quickly to get maximum power. I've seen elaborate systems for opening a ball valve but most use a sprinkler pump valve. The ball valve is difficult to open fast enough and the sprinkler pump uses batteries to open the solenoid. The batteries run down, and have other problems with peak current draw so that the valve doesn't open at all (more about that in the diary). I've also had trouble with leaking valves. The Spudgun Technology Center has invented a Supa Valve that overcomes most of these problems but it is very expensive ($90 up). Or you could use two valves at the same time as detailed on the spudgun technology center page. The solenoid valves are also expensive at about $25 a pop. Where an entire combustion gun can cost less, especially if you get the barbeque sparker from the garbage.
On to the advantages: Safety. These guns are inherently safer since no explosion takes place, meaning the gun won't explode in your hands if the barrel is blocked. If you shove too much stuff down the barrel then the worst that would happen is it wouldn't come out. Consistency, and control are big ones too. Since it's easy to control the PSI, it's easy to control how much power the gun will have when fired, and with the same pressure, the same power will be obtained every single time. These guns are also quieter, for what it's worth.
Generally combustion guns are more common because they are easier to make, less expensive, more portable and don't require an air compressor. The difference in power is hard to say because of the many different sizes of guns and different fuels etcetera. They're very close but one thing to note is that if you use a sprinkler valve on a pneumatic gun, it is only rated for a certain PSI, and the valve might fail if you go over that pressure.
The fuel used to power a combustion gun is often hair spray but it can be just about anything from naphta to acetylene. For the more powerful fuels like acetylene, a steel gun is a must to contain the pressure from the explosion. Personally I use propane for reasons detailed on the big combustion gun page. A common misconception among people very new to spudding is the more fuel put in, the more powerful the explosion. Just like in a car engine, the fuel and air mixture has to be just right to get the most power. Some fuels are more forgiving than others in that they have a larger range of flammability in air. If you put to much in or too little the gun won't go off at all or it will give a slower explosion.
Most guns are made out of PVC piping because it's supposed to be easy to get and cheap. It's important that one uses Schedule 40 PVC as opposed to the more common Schedule 20. The 40 pipe is thicker and stronger, and also more expensive. The reason I say PVC is "supposed" to be easy to get and cheap is when I went shopping for potato gun parts, it was hard to find. Maybe it's because I live in Canada, I don't know. I actually prefer to use ABS piping. The difference is that PVC is white or grey while ABS is black. Technically, ABS is a rubber while PVC is plastic. The most important reason for using ABS is that it doesn't shatter like PVC and it is more tolerant of temperature changes. In cold weather, PVC becomes brittle. PVC Schedule 40 is has a pressure rating meaning it has been tested to withstand a certain pressure (Here's a table listing the sizes and pressures ), while I don't know of any pressure ratings for ABS. One advantage of PVC is that it comes in smaller sizes than ABS. Generally speaking, I would be hard pressed to find an ABS pipe smaller than 1" in diameter without special ordering it. PVC pipes, on the other hand come in many more sizes, which would be very useful for smaller guns. Another difference is the way in which the joints are formed. In PVC, the joints expand in the hole. ABS joints dissolve the pipe on both pieces and then it re-solidifies. This is worth noting because when gluing PVC, sanding the joints would weaken them while in an ABS joint, it would strengthen them. Both joints are very strong unless you really screw up making it and you don't need to worry about them coming apart.
Lastly, a word on the origins of a spudding. Spudguns in their present form have been around for years but long before that there was the soup can cannon or tennis ball cannon. These were around way before PVC and ABS were invented and popular. They are a more simple form of a spudgun that consists of a bunch of metal soup cans joined together, usually with duct tape, with both ends removed to form a tube with one plugged end. A tennis ball fit down the tube but not all the way, leaving room for a chamber. There was a small hole near the plugged end of the pipe where you could put in a few drops of lighter fluid or gasoline or whatever in and then a daring soul would hold a match near the hole and hopefully, send the tennis ball flying. These soon became obsolete with the invention of ABS and PVC but the idea remains same.
This concludes this general introduction. With this basic knowledge I hope you will now be able to make sense of the rest of my page.