Industry maximum recommended specs for run-of-the mill 2 3/4” and 3” 12 gauge shotgun shells is 11,500 PSI. Magnum loads can go to 14,000 PSI. .22 Long Rifle is 24,000 PSI.
Quality schedule 40 steel pipe is good for a working pressure of 2000 psi, rated burst at 12900 in 3/4 inch that would be needed for a standard 12 gauge. That would be shaky with 12 ga loads that normally run to 12000 on the very high end, even with the sloppy fit between the “barrel” and “receiver”.
Cheap hardware store water pipe is a crap-shoot.
Sch 40 pipe is a crapshoot. I say this even though I built a slam fire 12ga shotgun about 50 years ago and still have all my fingers. It was an old idea even then.
A few years back I took another look at doing it again with stronger material. I could not find seamless 4130 tubing the correct diameter. That would have provided a safety factor of 2 or 2-1/2 to one for magnum loads from calculations. However, 1010 tubing is available in the correct sizes and gave a safety factor of about 1-1/2 to one for magnum loads (from memory). Didn’t do it, at least yet.
Seamless high pressure piping would be the thing to use. Still wouldn’t be too costly for the small quantity needed. I wouldn’t use cheap schedule 40 hardware store pipe for a hand held gun either. For the devices that are buried, of course, it doesn’t matter, they just have to work once.
There’s a whole bunch of folks who reload shotgun shells with low pressure loads for use in older guns. I have a 16 gauge recipe using Herco that is around 5000 psi, moving 7/8th of an ounce of shot at 1100 fps.