Do you mean ones that can be used at events, or just ones we have?
Me and my friends built a few. They're about 4' long. 3'x2" PVC barrel and a 1'x4"? combustion chamber. That's right combustion. The back screews off, you spray something flammable in the combustion chamber, close it up quick, and pull the trigger (a grill spark lighter). We shot potatoes about 2 football fields, and apples a little farther (they're jucier and slide better).
it's very similar to this one
http://spudtech.com/images/products/camo1.jpg