Hmm, well that explanation makes sense in a theoretical way jeepkahn, but I'll let you know It's very bold of you to take PVC to such pressure(except for someone just recently taking up the hobby). There are so many variables involved such as: An unseen error while solvent welding, a faulty/defective valve, improper appliance of Teflon tape to threaded joints, warped or weathered pipe witch voids the rating... the list goes on.
Also if you don't already know that rating is for water pressure, and according to my knowledge air pressure behaves differently than water pressure. Water has surface tension and air doesn't while under pressure. Water also has a much bigger molecule than air. I'm sure someone could better explain this than me and correct this if my statement is wrong.
Then again I have no clue about your background, and your respect for safety. I don't mean to be the Negative Nancy

but I'd also dislike feeding the media with another spud gun related injury.
My point is that the risk is too great. If you must build one then I would consider using a length of metal pipe as the chamber, and substitute anything else you can with metal. Even after this there's still risk, but there's always some risk
Sorry to rant but I wouldn't want you or your kids to be injured when it could have been prevented. Thanks for reading, good luck, TaTer