Depends on the tensile strength of the barrel steel and the intended load.
Twist and Crolle Damascus: about 55,000 psi
Winchester Standard Ordnance and other "cold rolled" Bessemer/Decarbonized steels: 60,000 psi
c. 1900 Fluid Steel (Siemens-Martin & Krupp Open Hearth Steel AISI 1021-1034): 75,000 85,000 psi
Krupp Fluss Stahl (Homogeneous Fluid Steel) was introduced about 1890 and by reported composition was similar to AISI 1045 which has an industrial standard tensile strength of 85,000 psi.
AISI 1040 (and modified), 4140 Chrome Moly (not used until after 1930) & Bohler Blitz: 95,000 - 100,000 psi
Winchester Nickel Steel and Marlin Special Smokeless Steel: 100,000 105,000 psi
Remington Ordnance Steel: 110,000 psi
And the really good stuff
Krupp Nirosta (1912 patent NIchtROstender STAhl 21% Chromium / 7% Nickel Stainless Steel introduced in 1913): 114,000 psi
Winchester Proof Steel (probably AISI 4340) introduced in 1931 for the Model 21: 115,000 - 120,000 psi
Krupp Spezial Gewehr Lauf Stahl (1895 Special Gun Barrel Steel): 138,000 psi
Bohler Antinit (Rostfrei Laufstahl chrome-molybdenum-vanadium introduced 1912): 138,000 psi
(NOTE: the tensile strength reported is for "cold rolled" barrel steel; all can be heat treated to much higher tensile strength for different applications ie. rifle receivers)
Courtesy of Hugh Lomas for English 12 bore Game guns with pre-1925 Proof: Chamber immediately prior to forcing cone - .105
Factory original 20 and 16 gauge small frame guns with Damascus barrels however may have a wall thickness of .090 at the end of the chamber, and .020 in the distal third of the barrel, making it critical to use loads that match the ballistics of those originally intended for use in the gun.
It is my opinion that any but obvious superficial pitting (since it is impossible to accurately measure the wall thickness at the bottom of the pit) at the end of the chamber with borderline wall thickness makes the barrel unusable (without the use of chamber inserts)
