That's where you cut the barrel a few inches after the reducer, add ANOTHER reducer, with a threaded bushing, to a threaded nipple to a sprinkler valve. The Charlie Foxtrot of fittings may hurt performance slightly, but it'll be better than the ball valve, (especially shooting something as light as ping-pong balls).
BTW, if you ever get the chance, fill one of those ping-pong balls with lead shot. We did this at my friends house and put a HUGE dent in a car hood, (none of our cannons have been able to do much damage to the car hood so far).
Nice job.
