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. Valves need maintainance and cleaing, some require batteries. Can make some cool fiery projectiles, loud.