I would suspect the octagon section isn't just tapered, but rather has a very gently "swamp" to it. Generally a straight taper will have a bulged appearance in the center and look bad. On of those optical illusions...
Thanks for bringing that up Jim. On the examples i've studied through photos, the 1760 Bailes in particular, there is a very mild swamp to the octagon beginning just ahead of the breech and seemingly disappears a couple inches back from the barrel bands. It's almost imperceptible until closely studying how the top rib is shaped for the first few inches. When mating the barrels the adjoining flats must be thinned somewhat to regulate the barrel set. This will have a tendency to negate the appearance of the swamp at the top rib but if one looks closely you can see it's still there.
Hank, are the barrels you are copying relieved inside the muzzle area like many birding pieces or are they cylinder throughout like a fuzee?
Those muzzles appear to maybe have end chokes.
James, Daryl, both are true cylinder bore. Thought about working an improved cylinder choke into one of 'em but decided not to.
Hank, have you done the math so you know what rough barrel diameter is required to machine octagon? Using your .875 at the transition would require a diameter of .947, at the transition, more if tapering from the breech.
As far as barrel wall thickness, your planned dimensions would work fine as long as you know the bores are concentric to the OD of the barrels. Is what I’m saying is, that while turning, if the barrels were turned elliptical then you could have thin spots. This can also happen in the final finishing or striking the barrels.
I do quite a bit of modern shotgun work, both steel and damascus, at least they were modern around 1890. These were black powder guns. I have the equipment to measure bores and wall thickness accurately. This can not be done accurately by taking the OD and subtracting the bore, divided by 2. I don’t want to get into what is safe, that’s a individuals decision, but do want to say that .010 anywhere in a barrel is not safe. In a barrel of the date I mentioned, I want to see more than .100, preferably more than .110, at the front of a chamber, which is usually the thinnest section of a chamber. Of course all this is on a barrel with generally a good bore, ribs, and so on. The thinest part of a barrel is normally 7 to 9 inches from the muzzle. I like to see more than .025 in this area. The barrel must get thicker from this thin section back to the chamber. Wall thickness on these barrels can be dramatically different radially at the same position in a barrel. This is generally from striking the barrel, and could be this way from new or because it refinished at some point.
The reason I bring all this up is so you are thinking through all the issues and what is considered safe for you and, of course, others around you when shooting.
Bob
I really appreciate your input Bob. Thanks! Yes, we did the math. Diameter of each band is 1.022". Raw blank diameter is 1.5". This is more than enough material to mill in the octagon to make it nearly flush to the bands and keep the proper width at the breech.
Given the physical properties of 1020 steel, 0.010" wall thickness is the
absolute minimum to assure a safe, functional barrel. This does not mean they will be made that thin. The step at the band is the thinnest part of the barrel and falls nearly 13" from the breech. Bore gauge says inside diameter of both barrels is 0.7495" and 0.7500" at 13" with negligible difference at 12" and 14". Indicated outside diameter TIR at 13" is -0.0000", +0.0005 and -0.0005, +0.0000" Hence, outside diameter of each barrel is concentric to the bore within 0.0005". Not even worth worrying about.
Now, back to wall thickness: Removing the extra 0.100" full length makes the barrel wall approximately 0.063" at its thinnest section. This more than doubles the burst pressure rating at 13". Does this mean the barrel is indestructible? No. If some dunderhead loads it with smokeless or short charges the wadding and shot or attempts to fire it with an obstructed bore then yes, there could be a problem. But is this not true for any m/l fowler? Or any gun for that matter?
Bob, the second pic of the trimmed barrel definitely shows some eccentricity. Have you measured the difference in wall thickness?
Because the barrel is centered on the bore at the muzzle when turned, it’s bound to have even walls there even if the drilled and reamed hole has some curve to it that will be revealed when some is cut off.
That's a good point Rich. Unless the bore is ridiculously angled end to end a turned barrel will be relatively concentric its whole length. A skilled machinist can recognize an off center condition and make adjustments to compensate for it resulting in a truly concentric barrel wall.
I've grown long winded and my apology to those that find this a boring lot of BS. But hey, i'm an engineer. If anyone has spent time working with one of those critters then you know how it can be from time to time.
Hank