Thanks guys.
As a paintball tinkerer I know all about what happens when pressure ratings are overlooked (got a few past injuries to prove it).
and like I said in the first post that picture is not drawn to scale. The chamber will be larger than the barrel but I can't make it too powerful because it ...

This will be my first pneumatic cannon so I have very little experience in this but I have a basic understanding (read: I have been reading the wiki and scratching my head for a few hours). It will be used for combating tanks in scenario paintball. It will be built primarily from pvc pipe primarily ...