Down south we call 'em tater guns. Mine is a whole lot shorter with a four inch combustion chamber reduced to two inch barrel. My gun utilizes a flint igniter with a good shot of ether. Thats right "starting fluid". I don't know the engineering specifics, however it will lob a good size loaded spud about 150 yards down olde "Brickyard Bayou" where I live. Thats some gun you got yourself. You might could call yous a Spud Skud.
The trebuchets people have started building are interesting too. I saw a site a few years ago where a group had built one to lob themselves into a river and there's a guy in England who launches pianos and cars with his. I wonder what the neighbors must think.