You might dig up a copy of the GQ article on Joel (the previous owner of spudtech) here's a link.
Propane or MAPP combustion in a closed chamber, in air, at atmospheric pressure peaks out at about 135 PSIA, 120 PSIG. This is the maximum pressure that a spudgun will ever be subjected to and would only occur if the round jammed in the barrel. In practice, even with very high friction ammo, the chamber pressure peaks out at 60~70 PSIG.
3" pressure rated PVC pipe is rated to at least 220 PSIG. Smaller diameters of pressure rated PVC have higher pressure ratings.
Joel has built pneumatic guns for the US gov't and various laboratories who needed guns that would fire large ammo (for example a 2"x4"x8' hunk of wood) at speeds in excess of a 100 MPH. Some of those guns were pressure rated PVC charged to 120 PSIG. If PVC will handle that then handling propane+air combustion at 1x is no big deal.
