I had a crazy idea. Precharged airguns use compressed air from a pump or a scuba tank to fill a cylinder on the gun. Would it be possible to fill that cylinder from a Propane or Butane tank to give similar pressure? (I know nothing about the relative sizes of gas molecules). If so, would the gun work? And, if so, what would happen if you introduced a piezo spark at the instant the trigger releases the gas to fire the slug? It seems to me that the burn would greatly increase pressure and velocity.
There is probably some great big snag which I can't see but I have no doubt somebody cleverer will put me straight.