Found some more interesting info: most shotgun actions are made from low carbon steel that CANNOT be through-hardened by quenching; only the case layer has enough carbon in it to respond to quenching; and 1450F seems to be the proper temp to cause that layer to harden, although 1650F is the right temp to cause it to absorb carbon with any degree of depth and in a reasonable time (hour or two). Carbon absorption is VERY slow at lower temps.
So....it seems that for initial case-hardening, you need to go to 1650F for a couple of hours to get the carbon to absorb. If the action is then annealed (eg for engraving), you only need to go back to 1450F for about 30 mins to reharden the case layer; and for recoloring, you don't need to go much above 1250F to 1300F and you don't need to hold it there, that temp won't affect metallurgy of the core or the case but it's enough to get fresh colors.
But here's the rub: the hinge pin may be made of different alloy than the rest of the action, ie higher carbon that WILL through-harden. So if the pin is still in the action, you need to make sure you don't screw up its properties by making it too hard thus risking brittle failure. That means staying below 1325F, or if you go above it, know exactly what alloy the pin is made of and heat treat accordingly (ie tempering of the pin may be necessary after quenching).
This is all conjecture based on limited reading so if there are any real metallurgists out there, feel free to correct what I have said.
