The build uses PVC pipes for both the barrel and the air ... which is nicely within the 280 PSI rating of the 2″ Schedule 40 PVC being used. Though, as they note, the connections in the design ...

The main designations of PVC electrical conduit are Schedule 40 and Schedule 80. (There’s also PVC plumbing pipe, but that’s an entirely different thing.) Here are the specifications for each ...