I've come across some seamless heavy wall tubing many times, stuff with the wall more than bore diameter thick, commonly called Shelby tubing. Most of it is 1018, but can be gotten in stronger alloys like 4140. A little study and it can be found.
I've been thinking of making a cannon for many years, and it never once occurred to me to weld the breech plug in, only to thread it at least a minimum of 1 1/2 diameters deep. What is common practice? I'm sure I'd never trust any weld.
Why are people calling it a "breach" plug? Is that the correct spelling?
The practical way to make a tube without casting, how to install the trunnions? I've kicked that one over for many years in my head. Thread it full diameter into just short of the bore (With a larger shoulder into a counterbore) and weld it? The amount of recoil force it will be required to withstand is fierce, and the end of threaded part should be squarely in hard contact with the flat bottom of the hole I figure. That's a lot of careful machining. I figure to make it contact the trunnion band shoulder at the same time as it contacts the bottom of the flat bottomed hole, to spread the forces over the larger shoulder of the trunnion while resisting expansion from the bore at the other end.
Rifling, there's no way I'd make one without rifling it, so I'd need to make my own rifling machine. The amount of twist, that's a science, not a guess.
The point is that as I mull these things over in my head I always end up admiring what others have done instead of doing it myself, but that could change. I have a large French lathe with a 3 1/8" hole through the headstock and 60" between centers.
If I do make one it will be able to take the pressure. No doubt about that part.