Some people take potato guns VERY seriously.
It is unclear how the SpudFiles.com sanctioned Spud Gun Contest is judged by their website, as access is restricted to registered members.
My guess though, is that the criteria is based more on measurable performance, such as projectile velocity and power, than on cosmetics.
Having said that, the appearance of the two semi-automatic winners (pictured here), are enough to convince anyone that the technology of the backyard potato-gun pistol have evolved a great deal.
A quick Wikipedia search defines the basic types of potato guns as the following: pneumatic launchers, regular combustion, and hybrid launchers.
Each one differs by it's combination of compressed air or fuel source and ignition.