Man, that ammo looks like it could tear your barrel a new one after several shots. Here's what I used to make shells with 6mm airsoft bb's that can be individually loaded like a 12-gauge shotgun. It should be just the same with these: Use a short section of scrap (1-2") from your barrel as a guide a...

I tried shooting a buddy with snow packed in an ABS combustion cannon (4"x24" chamber to 2"x24" barrel). In retrospect, I'm glad the recoil made me narrowly miss because the heat in the chamber from previous shots made the ice melt into one solid block.

I think the type of fuel should also be considered. Certain types (gasoline, acetone... off the top of my head) cause PVC to become soluble, so no matter if the PVC is pressure tested or if it's very well-built, you'll be risking a busted chamber.