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8620 is low carbon steel designed for a deep case (Up to .060). 4140 is high carbon steel. Seems kind of dicey to quench in water. There is not much control using water. I never knew any other quenching method to color case. I know 4140 can go up to 70 RC under the right conditions quenching in water. Maybe salt brine? There are military receivers made of high carbon steel but none I know of are case hardened. That seems odd because of the wear factor. I suspect a strength problem is involved there. I heard rumors that a Chech or Polish Mauser used tool steel in the receiver, but I never nailed that down.
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