Lt. Col. Brophy's fine book on the specs. for L.C. Smith shotguns calls out the AISI grade for different parts, also the respective heat treatment, resultant hardness expected, etc. I wish someone had done this for Parker, Ithaca, etc. All these fine older American doubles were "double fitted"- the first fitting- soft fitting or "in the white", the final fitting after the case hardening process- so a somewhat ductile lower carbon machinable steel was required- and AISI 1018-1020 was used.
The old "bunkum" about barrel steel grades, other than imported Krupp Flustahl (fluid steel) and Whitworth Fluid Compressed steel barrels=- Parker using terms like Vulcan, Titanic, Acme, Peerless- Smith with Armor, Nitro, Crown- etc- when neither company made their own Nitro proof steel barrels, was just good advertising-- Hunter Arms. marketing lads were way ahead of their time- if you'll notice all the aforementioned barrel steel grade names have FIVE letters- as did Kodak and Zerox-- huumm.
EDM is right about old "Hank the 1st." Ford and the AISI 6150 Chrome Vanadium steels used first in European race car engines-another reason he developed the Rouge River complex and his own steel mill perhaps.
Winchester broke away from the older "double fitting" when it brought out the great M21- by using AISI 4130 and 4140 they were able to machine the components after hardening-- The M21 may not have the "cachet" of the other older American doubles, but as the now late M. McIntosh once said- they are "Hell for stout"--
