Seamless is good stuff. Now you're talking engineering. What material is the seamless, what strengths are you building for, and what safety factor do you envision? Where are the stress points? (at the muzzle, any interior sharp corners, fuse hole, etc.)
OK, so you're not a mechanical engineer. Neither am I. So what's out there that is of the same caliber, charge, bullet weight, bore style (smooth or rifled) etc. that has been in use for some time with out catastrophic failure and that is recognized by organizations (as the N-SSA) as being of good design. Might be a good design?
For example. If you look up 4130/4140 you'll find it (in prehardened state) has about 100,000 psi tensile strength (applicable to hoop strength). With some hardening it goes UP in strength from there but is harder to machine. Contrast that with a mild steel, say 1018 may have 60,000 psi tensile strength (annealed).
Probably most of this discussion comes from my having seen a cannon explode (while standing with several others not more than 15 feet behind it). I build from 4130/4140 for the added strength and not wanting ANYONE to read about a catastrophic failure in the national news.
In any event, if you're designing, you need to have the confidence that you're making a cannon or mortar and NOT a BOMB. Don't ASSUME; but do make reasonable assumptions and check them with knowledgeable people. If you're out of the mainstream, folks will tell you. (Not just the ones that tell you you're crazy for building a bowling ball mortar.)
