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I remember hearing the opposite, 20 years ago when I made one–that PVC was preferable to ABS because you couldn’t readily buy ABS with any sort of pressure rating.

It didn’t even occur to me that ABS pipe is pretty much only used for drainage and vents. I think it’s still probably decent to have unrated pipe tear smoothly rather than rated pipe—over pressured—shatter?

I guess the pro move is to either (1) fully characterize the amount of pressure made from a random shot of hairspray, then choose a PVC pipe to accommodate, or (2) put a larger ABS sleeve over the inner PVC combustion chamber to contain the shrapnel.

That's why you do a pressure test at twice the working pressure, with the thing under a blanket.

This should be repeated every year.

We didn't know this and I'm realizing now were incredibly lucky!

Oh shit - we did not know. I guess we got lucky too!