Thought I'd add this information regarding vintage double
ACTION steel.
Plans & Specifications of the L.C. Smith Shotgun by William S. Brophy contains an undated but likely post-1913 Materials Specification chart indicating AISI 1020 Carbon Steel for both the frame and barrel.
However, under the Featherweight drawings dated Feb. 19, 1929 the lug specifies a forging of 1020 steel, but the barrel is Steel App. 40 Carbon, likely AISI 1040.
Just came across this of interest
July 1, 1920
American Machinist published an Ordnance Salvage Board Surplus Property Sale of almost 75,000 pounds of Spec. Shape Gun Steel from the
A.H. Fox Gun Co. with C .15-.25%, Mn .5-.7% and S & P < .06% = AISI 1020 and likely frame steel
https://books.google.com/books?id=ezRMAQAAIAAJ&pg=RA1-PA409&lpg A.H Fox introduced
barrels with Chromox High Pressure Fluid Steel (chrome-nickel and vanadium steel) in 1912.
AISI 8620 (a .20% carbon and chromium, molybdenum, nickel low alloy steel) eventually became the standard for frames; it is easily carburizing and machined when annealed. High dollar modern doubles mostly use 4140, or better.
