WARNING: This is sort of "thinking online" and my mind is a scary place to visit!
I think I may change my plans a bit. I purchased some cow magnets today that are made from square bar stock and have a ceramic coating on them. When applied to a flat ferrous metal surface, they seem to hold with about twice the strength of the round magnets. I would assume that removing the ceramic surface would make them even stronger.
As my stock is already set up for an octagonal barrel, it seemed to me that it would be easier to use an octagonal barrel. Also, the magnet would hold better on the bottom flat of an octagon than it would on the bottom surface of a round barrel. I have a round barrel fragment that has a rusted exterior but a fairly decent bore. It is long enough to get at least two and maybe three 5 1/2" barrels out of. The exterior diameter matches the exterior diameter of the T/C Hawken barrel across the "points".
You guys know where I am going with this don't you?
I would cut the new barrels, face them off and crown them properly. Then get the bastard out and start draw-filing the new barrels into octagons to match the original barrel from that stock. At this point, I would leave one barrel at .54 caliber, polish one out to a .56 caliber smooth-bore, drill & ream the remaining barrel to sixteen gauge. Alternatively, I might make one smooth-bore sixteen gauge and one 16 bore rifled barrel. As the muzzles are planned to extend beyond the stock, I could also continue to draw-file them and make them half-round barrels... or maybe just quarter-round barrels. Pretty sure I can get access to a lathe, so could turn the round portions. At some point, breech plugs would need to be installed.
Now, at this juncture, I still have a great deal of research to do. It's been many years since I last stood beside a lathe, but I think I remember most of what I learned in 1975... though I am probably a little rusty on calculating cutting speeds. I've seen reams the proper size to do 16-gauge, though to go from a .54 caliber rifled bore to a 16 gauge smooth-bore, it probably ought to be a piloted ream. Could probably also just drill it out to the proper diameter for reaming. A drill bit of that diameter would not wander all that much as the work is already sort of "center drilled". Some of this work could also be hired out I suppose.
I'm guessing I'd probably be better off fitting the breech plugs before machining the bores. Not sure just how shotgun breech plugs are fitted, but I'd bet that they are not bored straight through at bore diameter before cutting any internal threads.
I'd also guess I'd be better off machining the bores before any draw filing.
Then there is the rifling project. I've seen a few videos on home-grown button rifling using hydraulic rams to pull the button through the barrel... and this would be a pretty short barrel. It didn't look all that complicated.
The life-plan is to retire in about three years. That is probably just about the time-span I will need to complete this project... which is perfect as I will then have plenty of time to actually shoot the finished product before I croak and have to give it to somebody else to play with.
Stay tuned...