Just for perspective, AISI 8620 is a chromium, molybdenum, nickel alloy steel often used for modern shotgun frames, and is easily carburized and machined when annealed
Carbon - .18 - .23%
Manganese - 0.7 - 0.9%
Chromium - 0.4 - 0.6%
Molybdenum - 0.15 - 0.25%
Nickel - 0.4 - 0.7%
Phosphorus - 0.035% max.
Sulfur - 0.04% max

Nice short version regarding the components & alloys
https://www.linkedin.com/pulse/21-chemical-elements-effects-steel-mechanical-properties-jeremy-he/

The Hunter Arms frames had a very high Sulfur concentration, which improves machinability but lowers transverse ductility.

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