You need a working knowledge of mechanics, hardware, construction and physics to know what you are doing, and it is good to know simple electronics for the triggers to some guns. Prices go higher as the diameter and size of the gun increases. 10' 90 deg. Elbow 45 deg. Combustion cannons usually have a large chamber which reduces to a smaller diameter pipe, from which the potato is launched.
Combustion is achieved by spraying in a type of propellant, usually hairspray.
A flint or electric lantern igniter is used to generate a spark, which ignites the gases, propelling the potato, which can exit the barrel baked. Pneumatic cannons are completely different, and much safer. These cannons have two chambers, one in which air is compressed, and another in which the projectile is shot from. Between these two chambers is a diaphragm, a valve of some sort. When the valve is opened, massive amounts of high pressure air force the potato out.