This takes me back. Our version fired tennis balls, using 3-4 soda cans (back when you could remove both ends with a can opener) with a tennis ball can on top; that way the ball would not travel all the way down the barrel. Duct tape, of course, held the whole thing together. Arm by squirting lighter fluid into the touchhole at the base, then swoosh the tube back and forth to aerate. Hold tube at angle with base resting on ground, pop in ball, light with strike-anywhere match. Done correctly, it would fire a tennis ball several hundred feet with a hearty roar and (visible at night) an impressive sheet of flame from the muzzle. Side effects of correct firing included lost finger hair from touchhole blowback, the disintegration of the gun as the duct tape softened from the heat of repeat firings, and on at least one occasion the immolation of the tube and its subsequent loss from stomping on it to put the fire out.
It still amazes me that we suffered no casualties from the above-described activity, or from the various permutations of model rocketry we dabbled in (Armor piercing? Check. Stuffed with firecrackers? Check.). On the other hand we inflicted no casualties either, so we had that going for us.