Thanks for all the information. I was tempted to just send the darned thing back and say to heck with it.
I did not realize that creating a flat on the side of a round barrel was done for flintlocks, as Jeff pointed out. Up until now, I've just been shooting percussion guns.
Also, while looking at dimensions, as he and others suggested, I realized the rather large flat is an illusion. It's a smaller flat filed into the barrel, with a larger peice of steel very smoothly shaped and soldered into place into the flat. This provides a larger flat surface, and looks like part of the barrel, but actually is larger than the barrel. At first look, I thought the large flat I was seeing was actually barrel, since it almost disappears into it, but the filed area of the barrel is much smaller.
It's actually a darned interesting setup, giving you a nice flat to work with, while minimizing the actual removal of material from the barrel.
That's what I love about black powder: it's a constant learning experience.
I haven't gotten over to get my calipers, but the barrel at the bore end is nice and thick. Given that the bore is tapered (and thus even thicker at the breech end) and the cut smaller than I originally thought, I think the measuring will go well.
Thanks for all the input! I would have likely just sent the gun back, and it might have been a long time before I realized just what a good setup this actually was!
If anyone is interested, I can get pics and send dimensions. I didn't want to disassemble it if I was going to return it, but i think the rough dimensions I'll be getting should alleviate my concerns and I'll pull it apart then.
Dave