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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.
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.
1020 was easily machined and case hardened, but once the case wears, 4140 chrome moly it ain't.
I would be very interested in other U.S. maker's documented action steel choices
