Here are some minimal chamber wall thickness, at forcing cone end of chamber, I've measured for shotguns either in proof (bore diameter in proof 9" from the breach) or (for a very few of those listed below) so deemed by Kirk Merrington. All also have min wall thicknesses of ≥ 0.020, preferably ≥ 0.025.

English:
James Purdey (1898) 12b., 2.5" chambers: ≥ 0.100.
James Woodward (1909) 12b., 2.5" chambers: ≥ 0.098
James Woodward (1909) 12b., 2.5" chambers: ≥ 0.103
Boss (1897-8) 12b., 2.5" chambers: ≥ 0.090
Westley Richards 12b., 2.5" chambers: ≥ 0.090
James MacNaughton (1895) 12b., 2.5" chambers: ≥ 0.095
Wm. Pape (1898) 12b., 2.75" chambers: ≥ 0.100
EM Reilly (1887-1904) 12b., 2.5" chambers: ≥ 0.091
Henri Egg (1870) 12b., 2.5" chambers: ≥ 0.108
WH Monks (1875-87) 12b., 2.5" chambers: ≥ 0.116
WC Scott (1905-6) 16b., 2.75" chambers: ≥ 0.105
Francotte (1938) 12b., 2 5/8" chambers: ≥ 0.098
Westley Richards (1905) 20b., 2.5" chambers: ≥ 0.098
WW Greener (1922) 20b., 2.75" chambers: ≥ 0.098

Belgian:
Francotte (1894-5) 12b., 2.5" chambers: ≥ 0.085
Francotte (1896) 12b., 2.5" chambers: ≥ 0.085
Francotte (1930) 20b., 2.5" chambers: ≥ 0.095

French:
Verney Carron (1950s) 12b., 2 5/8" chambers: ≥ 0.100

German:
Wilhelm Brenneke (1902) 12b., 2.5" chambers: ≥ 0.110

If we are concerned about shooting safe vintage guns, shouldn't we be concerned about the thickness of the metal where the chamber meets the forcing cone, where the pressure is higher than further down the barrel (where we look for ≥ 0.020") and how low (thin) can we go? Looking at the data above, is a 16b. with a chamber wall that is 0.076"-thick at the forcing cone OK (even though the barrel wall min thickness is 0.030") safe to shoot? The answer to this question seems to be "yes" as this Parker had clearly been shot quite a bit and still has factory specs. Is there actual data somewhere on this point?