We have fired our 4" x 18" chamber, 1.5" x 10' barrel ABS spudgun using a full charge of oxygen and acytlene from a welding outfit and the gun still works fine. Mind you, it was a really scary experiance that I don't care to try again. Realize that this is pure oxygen I was using so it would be AT LEAST 5 times more powerful than any other propellant you could other wise use. Not recommended. Won't do it again.
After sizing the potato by shoving it down the barrel we pushed it back out then I cut shallow spiralling channels down the sides of the potato to give it 'rifleing' since I could not rifle the barrel. That made a HUGE improvement in the distance the potato went. Imagine a quarterback throwing a pass where the ball tumbles, then imagine him throwing a perfect spiral. Now you get the picture.
Using a rifled potato and oxygen/acetylene the potato went so far we couldn't see it at the end of its flight. 1000 yards plus. I won't do it again. Jail time comes to mind.