Now that this thread has been completely hijacked by an Ego contest....
When I built my air cannon in high school, I had the help of my father who has worked in Water Treatment for about 30 years, so thankfully I was blessed with knowledge from above about pressure ratings.
I used 4 inch Galvanized Steel, and typically ran it at 135 PSI.
I was able to launch aluminum baseball bats about 250 yards with about a gallon of water in the barrel.
Enjoy some very dated pictures:
