Just finished examining the barrel. I used a tiny drop light I have that has a white light and is the diameter of a thin pencil. The light has a hole in one end which I attached some stiff wire. The bore is heavily corroded. If one wanted to use the barrel for shooting, it would HAVE to go to Bobby Hoyt for re-boring and likely a steel liner. IMHO. (It's tempting LOL) Fortunately for me, I won't be going this route.
So the barrel is 65" long. Running the bore light down the barrel, it stopped at about 59". Just at the beginning of the swell. Which would be just about right where the narrow section begins. The barrel is so long, I can't actually see it, but I'm sure I touched it. An iron rod makes the correct "clang" when bouncing it off the narrow section. Also, a piece of thin wire and a pipe cleaner through the vent hole produced no evidence of powder. Just rust/dirt. So, I'm confident the barrel is not loaded, and the breech design is what I was looking for. I plan on cutting the barrel in two about 6" in front of the beginning of the narrow section. That way it will show the transition to the nominal cylinder bore from the rear of the breech. The breech wall is extremely thick. Once the barrel is cut in two, I'll take some more picks. Maybe another surprise. LOL
I have now confirmed I have the right barrel for the project. So I'll check with Flint62 and ask if he's still a go. And I'll pull the barrel off the stock and get to work with the hacksaw. (Unless I find someone with a bandsaw for metal) LOL
Rick