A combustion cannon is a cannon powered by the combustion of flammable fuel-air mixtures. A basic combustion cannon is the simplest one to build, and is therefore often the choice for first time builders.
Combustion powered spudguns have four basic elements:
In order to fire, the operator loads a projectile (usually a potato) into the barrel, adds fuel to the combustion chamber (for example aerosols or propane), and triggers the ignition source (e.g. a piezoelectric BBQ ignitor). The fuel should then ignite, create hot expanding gases, and force the projectile out of the barrel. Distances vary greatly depending on the type of fuel used and the stoichiometry of the fuel/air mixture, the size and chamber to barrel ratio (C:B ratio) of the launcher; 100 metres is a common distance.
Improvements on the basic combustion launcher include optimized C:B ratios, metered propane injection, chamber fans, multiple spark gaps (spark strips), and high voltage ignition sources (flyback circuits, stun guns, camera flashes, etc). Combustion launchers are mechanically simple, but are less powerful in typical situations* compared to a pneumatic launcher of the same size. Potato guns are often painted to increase their aesthetic value and to cover unsightly primer stains. Krylon Fusion, a type of paint specifically made for plastics, is the most common type of spray paint used on potato cannons.
*) the mass of the projectile is high enough that the faster cannon is not "solely" decided by the speed sound in the gas and the flow of the valve, and low enough that these things still matter. This will include almost any projectile you'll ever shoot.