I quote "Instead of following the usual practice of utilizing case-hardened material for the frame, Winchester employed chrome molydenum alloy steel, the famous Winchester Proof Steel, having a tensile strength of 90 tons per square inch" Ned Schwing, WINCHESTER´S FINEST THE MODEL 21, P. 22.
"When Winchester decided to built a double barrel side by side shotgun it wanted a set of barrels that were strong and accurate. The result was barrels forged in the Winchester plant from specially heat treated alloy steel having a tensile strength of 115,000 to 120,000 pounds per square inch and an elastic limit of aproximately 105,000 pounds per square inch. This was more than twice the strength of the usual barrel sreel."..... "Model 21 barrels began as solid core blanks of special chrome molybdenum alloy developed by Winchester metallurgists working with the steel company´s metallurgists to devise a formula strictly for Winchester barrels" Ibid., p. 57.
Schwing drescribes the process but does not goes on technical nomenclature.