Oh my. Dean I only impersonate a metallurgist on gun forums.
Extremely short version is that for Carbon Steel (AISI 10XX) the last 2 numbers are the Carbon content. Parker Titanic Carbon was measured at .32% = 1030 with an industrial standard tensile strength of 76,000 psi
http://www.engineeringtoolbox.com/ai...em-d_1449.html
Carbon steels
10xx Plain carbon (Mn 1.00% max.)
11xx Resulfurized
12xx Resulfurized and rephosphorized
15xx Plain carbon (Mn 1.00–1.65%)
Relative to gun barrel steel, the higher the Carbon content, the greater the tensile strength.
Summary of “Cold rolled” barrel steel tensile strengths. All can be heat treated for different applications (rifle receivers) to much higher strength, and yield strength matters also.
AISI 1005: 40,000 psi
Twist and Crolle Damascus: about 55,000 psi
AISI 1015: 56,000 psi
Winchester Standard Ordnance and other "cold rolled" Bessemer/Decarbonized steels and AISI 1020: 60,000 psi
c. 1900 “Fluid Steel” (Siemens-Martin & Krupp Open Hearth Steel AISI 1021-1034: 75,000 – 85,000 psi
AISI 1140: 85,000
Krupp Fluss Stahl (Homogeneous Fluid Steel) was introduced about 1890 and by reported composition was similar to AISI 1045: 85,000 psi.
AISI 1040 (and modified), Bohler “Blitz”, 4140 Chrome Moly (not used until after 1930s): 95,000 – 100,000 psi
Winchester Nickel Steel and Marlin “Special Smokeless Steel”: 100,000 – 105,000 psi
Remington Ordnance Steel: 110,000 psi
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
Phosphorus increases strength and machinability, but can embrittle steel, esp. if cold (ie The Titanic).
Sulfur increases machinability, but if high Sulfur/Manganese ratio leads to Manganese Sulfide inclusions.
Nickel & Chromium increase corrosion resistance and hardenability.
I think we have good data that the Belgian produced "rough forged tubes" used by U.S. double gun makers prior to WWI were mostly AISI 1018 - 1030, possibly rephosphorized, with Decarbonized Steel on lower grades.