What is a spudgun?
There are many different names for spudguns. They are called potato cannons, spud launchers, potato guns, spud cannons, and a host of others. Whatever you decide to call it, all of the aforementioned describe the same type of device: a device used to accelerate a potato down the barrel, and shoot it some distance. While it may offend some purists, the humble little potato is quickly losing favor to other ammunitions such as golf balls, tennis balls, and other cheap, uniform projectiles.
There are two common means to launch projectiles from a spudgun. The first uses the energy stored in a compressed gas and the second, more common method, uses the energy released by combusting fuel vapors. Also, there exists a less common combination of pneumatic and combustion launchers, a "hybrid", that combusts a pressurized fuel-air (or fuel/oxidizer) mixture.
As more advanced concepts continue to be designed and built, the general definition of a spudgun has become a homebuilt launcher that uses expanding gases to propel a projectile, and does not use any compounds which contain their own oxidiser for the actual propulsion.
The object of any of these varieties of spudguns is to propel projectiles in the direction of a designated (safe) target, for recreation and sometimes science experiments.
Common Ammos include: potatoes, (obviously), small citrus fruit, tennis balls, golf balls, bb's, or marbles.... in fact, pretty much any spare vegetable or commonly available object you can ram down the barrel. This is part of the beauty of spudguns.
Some spudguns are very simple, inexpensive designs, such as the aerosol-spray-powered combustion or ball-valve and bike pump operated pneumatic. Some can get complicated and/or expensive, such as large home-made piston valved pneumatics and propane metered combustion guns. A decent potato gun can be built for as little as $15 or $20 at the local hardware store, ranging right up to several thousand with parts from across the world.
When proper safety is practiced, spudding is no more dangerous than other common hobbies, such as model airplanes. Though as with any activity that can be dangerous, safety should always be your number one priority. So please, take the time to read the safety section.