speaking of potatoe guns when I was making them I would aim for the barrel to be twice as long as the combustion chamber, and also getting the fuel and air mixture right was critical...as far as fuel I never used propane...in the beginning I used Right Guard pit spay but it can leave a funky smell and also the less you use the better...my fav fuel turned out to be the spray Orange engine degreaser( it shares some of the same ingredients as gun powder mostly Carbon and Sulfur)

here is a link to some fuels that have been tried

http://www.aaroncake.net/spuds/boom.asp