BTW, the potato-gun in question has various breech-blocks for either Compressed-air, or air-propane propulsion... Has a 'monopod' that sockets into the truck's ball-hitch for handling recoil.

[url=http://www.youtube.com/watch?v=gwze3L3chQw]Although a black-powder eight-inch mortar sounds neato.[/url]
-[url=http://www.youtube.com/watch?v=tWtbkBNAJvE]Because unburnt blackpowder = tracer round[/url]