LOL, be careful. That's messing with cruelty to animals and I believe potato guns are no longer legal. And least that's what I heard when I had the urge to build another one. Best one I made back in the day utilized a 4" PVC combustion chamber, which narrowed down to a 1.5" barrel. That thing did things.
What do you use for the ignition? The first ignition that we used were 2 screws into the barrel and it worked sort of like a spark plug - we used the insides of electronic lighters to make the spark. Then we upgraded to a lantern lighter with a replaceable flint because the screws would get coated with hair spray and stop arcing. Fun times, we used to buy potatoes by the 50lbs bags. Fuel, we mainly used hair spray, that's what kept us out of trouble (cops couldn't do anything because it wasn't gasoline), but tried pure acetone a few times.
Pete