When I built mine in college, I used a 22kv power transformer instead of a piezzo ignitor. I could jump an arc an inch and hold it there. Never failed to fire.

Because of this, I used 2 4" x 2' pieces going to a y adapter, then to a 1.5 reducer to get to the 3' barrel. Wiring both combustion chambers together in series, I could shoot a potato about 300 yards. Great fun.