Instead of propelling the marbles with pneumatic power, why not fire?

not an explosion, just a constant torch. Propane is fed into a relatively small chamber (1in) which is attached to the barrel. At the same time, nitrous is fed into the chamber as a slow leak(via screw valve), just enough to give the chamber enough oxygen for a continuous flame.
My idea is not to make it go BOOM!, but to make it go WHOOOOOOOOOOSH, thus propelling marbles that are fed into the barrel.
ps. the gun shall be all metal.