[FourPenguins]

I've decried the use of PVC tanks before, and I stand by that. I would never use PVC for storage of any more than atmospheric pressure. I've seen what it can do, and i know someone with the scar to prove it.
However, the odds of damage to the barrel of the cannon are fairly low as the air is bound to follow the path of least resistance. In a sealed tank, this would be any crack or weakness in the tank. In an open barrel or the pipe to an open barrel, this would be the opening in the barrel, with only the resistance of a light shirt.
Is there some risk of a leak, a crack, or a blowout? Yes. Would that risk exist with any material? Also yes, although perhaps it would be a smaller risk with heavier/stronger materials. Like everything, it's a case of acceptable risk.