This guy is absolutely correct, those fittings are NOT pressure rated and pvc shards are SHARP. Also your valve us a HUGE air restriction because its tiny and it opens REALLY slow because its a plastic ball valve. If you liek this kind of thing head over to spudfiles.com, thats where real homeade guns like this are, not nerf.
What ever you do, do not pressurize that cannon. Those bell reducers you have used to go from 3" to 1 1/2" are a special kind of fitting. They are called DMV fittings or drain waste vent. They absolutley not pressure rated and you have basicly built a bomb. Next time when make sure that your fittings and pipe are not DMV.
And you guys can stfu about the metal, pressure rated PVC pipe is safe even beyond its rating so don't go beyond it, and don't let it get brittel in the cold and you will be fine.