I worked in a steel fabrication shop and there were lots of seamless steel pipe that one could make a muzzle loader out of real easily. Put a threaded cap on it, drill a hole for the touch hole, and you were in business.
But then, you could probably also make a cartridge firearm out of a piece of triple wall seamless. A “slam fire” shotgun.
None of it I worked with was of weak cast iron.
Firearms are actually EASY to make.
The Tyrone Power movie AN AMERICAN GUERILLA IN THE PHILIPPINES showed how it was done using two pieces of pipe, and a pipe cap.
About 30 years ago there was a company advertising in SHOTGUN NEWS, that made and legally sold such firearms.
Here is an interesting link to a “hit” with a slam fire shotgun.
https://www.the-sun.com/news/3840992/video-gunman-shooting-homemade-slam-shotgun-wild-west-gangland/
Bkmk