Greg, I've seen that photo before and I think it is of great help. Since it doesn't provide a measurement at 8", and since that is a common measurement area, would it be reasonable to estimate that it would be .070" at 9" from Breech? And .081 at 7.5" from the Breech? Thus, at 8.25" from Breech it would be
.075"? I could extrapolate what 8" would be with a spreadsheet ASSUMING that loss of metal to the walls is consistent in the gaps on the drawing above.

I've heard before that you want to be at .045" to .050" at 8" from the breech but I could never identify the reference that drew that conclusion, I merely am repeating folklore.

Above all, I want to:

A.) Be safe
B.) Buy safe, usable guns that may be overlooked as having too thin of a minimal wall, provided that it truly is a safe gun where the metal really counts. (e.g. more metal further towards the breech, why would I care if it is .020" in a cylinder bore 2" from the muzzle?)

The other question I have for the group is, does math help tell the story? What I mean by that is that I've heard some guns (e.g. old Boss) were made very thin from the onset. I have a Boss hammer gun with .025" minimum walls and the bore diameter is pretty close to .729, and it's marked as a 12 bore on its original proof. Wouldn't the math involved say that if it left the factory as a marginal bore diameter of .729 and therefore it is likely it was born fairly close to a .025 minimal wall thickness?

What is the most common bore diameter for a vintage English 12 bore (actual bore) that was proofed as .729/12 ? Would they have been likely to be about .725 on average at factory new? .720? Or .728"?

The reason I suppose this is important to know is in order to determine just how much of the bore has been removed from honing over a century of use and what was the likely reduction in metal over that period of time. If someone could provide a reasonable estimate of how these .729" guns were really bored, we'd probably be able to estimate how the wall thicknesses probably were originally too, compared to where they measure now.