In closing
this is important.
This illustration was part of the Birmingham Proof House Trial Report of 1891
https://docs.google.com/document/d/1...hfDxmrNAU/edit
Three 30” rough bored 12g barrels of 39 different materials (117 total) were received, and the barrels turned and bored to standard dimensions, without cutting the chamber; each barrel was fitted with a breech plug and touch hole and loading was done from the muzzle.
The wall thicknesses do NOT represent standard double gun barrel dimensions.
Note there is no chamber cut nor choke.
For the record, I've found no engineering drawings nor specifications for wall thickness for any U.S. maker. Fortunately now we have LOTS of numbers of presumed unmolested barrels with which to make rational recommendations.