I just finished to delete dents and stratches from receiver for my DR project, there were some minor case color traces in some hidden places...
I have to say case hardering do not ad any significant aditional hardness to the receiver I believe, the only reason of case coloring-hardering - to protect steel surface from small dents and stretches and that's all.
You could make it, case-coloring, at low temperature (it will be smarter) or at high temperature. Why smarter?
IF the receiver steel is something just a little bit higher than mild steel in % of carbon content, you risk to turn steel to fragile composition and without the following steel normalizing or tempering.
Broken re-case coloring receivers, which we see from time to time is result of wrong case coloring mid carbon content steel at high temperatures OR many times re-case coloring, when low carbon steel turns to something else fragile and large granular.
