d...The critical temperature was never surpassed back then and is not now days by experienced smiths...
On one of your above statements you were absolutely correct, I have "Never" cased a shotgun action. I did serve a certified 4yr machinist apprentiship which included a course in heat treatment of metals. I worked another 31 yrs in machine shops & was in on a lot of heat treatment, just not gun parts.
I can "ABSOLUTELY ASSURE YOU" if the "CRITICAL TEMPERATURE" is not reached the part has not been "CASE HARDENED", only colored. I can equally assure you the gunmakers of old did not go to all that work, expense & time doing all that fancy packing of the parts in charcoal which you have mentioned "JUST FOR THE COLOR", No they Case Hardened Them. They knew there was a high possibility of dimensional changes & did all in their power to minimize it. "If" you didn't go to the critical temp most of that would be totally unnecessary.
If you anneal parts, then heat them "BELOW" the critical temp of the skin then you have Destroyed all the hardness of that original case & insofar as I am concerned have "RUINED" that gun. As Lily Tomlin used to say in the big rocking chair "THAT"S THE TRUTH".
Of course "I DON"T HAVE ANYTHING TO SELL".
PS; Ask any metallurgist what it takes to "Harden" a piece of steel. Hardness is not produced by quenching unless the part is over its critical temp. Just because a piece of 1020 steel happened to be forged into the shape of a dbl shotgun action it doesn't harden any different than a part for anything else. This is a basic metallurgical fact & I was a metal worker.
