My understanding is that at least a couple of the well known CCH outfits will accept 4140 to CCH. It doesn't apply to classic receivers, but they manage the process on an alloy that can deep harden.

I noticed the 1.9mm depth of case for 8620 was at much higher temp. and much longer duration than typical reports for gun part CCH. So chances are the significant sounding depth does not actually happen if 8620 is used in one of those repro. single shot receivers.