Im not exactly sure how much tempering the parts @ 400F for an hour does, for the most part it seems like all it really helps with is completely drying the action out. Id be hard pressed to believe that it actually reduced any residual stresses in the action(but maybe the small parts), simply because its not that hot. A good test would be to harden two parts, temper one at 400F, then see if there is a difference in hardness, breaking, flexing, etc. The L. C. Smith engineers letter says that they used to put the freshly quenched actions on top of the flipped over empty crucible to completely dry them out. Maybe this also serves as somewhat of a tempering process? Ive run test actions as hot as 1450 for two hours and quenched (exact same setup as Dr. Gaddy, sliding shutter system and all), and experienced minimal on-face warpage and minimal tang warpage, but, and heres the kicker, 2 out of ten had horrible (1/16" off face on one barrel) and horrible tang twisting and bending (down I believe). I highly doubt that any shop would be able to stay in business if their success rates were this poor.
Anyways, as for properly blocking, this is extremely difficult to do and very time consuming (most specifically for doubles). Its sounds easy, but here is what goes on, and this is prior to shielding!
On all double guns, you have a number of "areas" of concern when re coloring. The frame has two parts of concern, the "front" and "rear". The "font" area (water table and breech face) are rather large, making up the majority of any double guns action. (this is just box locks we're taking too, dont even get me started on sideplated guns). The "rear" of the action is the upper and lower tangs (very thin, and only attached to the frame in essentially one place each, and only attached to each by two screws usually).
Step one is assemble the stripped and polished gun, blocking the hammer slots, etc with perfectly fit steel blocks, as well as perfectly fit screw braces (steel tubing) to prevent any of the tang screws from bending, and it also helps to keep the tangs in proper alignment. This does not however prevent the tangs from bending down (or up) simultaneously (or twisting left or right! which is not good when trying to put an action back in the stock and all of a sudden you have 3" of drop, are now cast off, and have a rather large gap at either the top or bottom of the stocks head depending which way the action was quenched). So, you can try to brace the tangs all you want, but your still risking a lot of warpage, and that is true at any temperature.
Second is the "front" or actual frame. You have a large lug recess, which you can shape a piece of steel to nicely fit in. That does not however solve the issue of completely twisting the action at the breech, essentially destroying the gun because it is so off face. You can make up a jig, similar to the one Gaddy used (which can be noticed in his DGJ article. Its a massive block that sits in the lug, drilled and tapped for two screws to sit firmly against the face of the action), but this still does not necessarily put you in the clear because the breech can still twist in a number of ways away from the brace. That and you will lose colors where the brace touches the frame, something that you wont see on any of the "pros" jobs.
You could even go "overboard" (there is no such thing when it comes to color case hardening) and custom machine something to integrate the locking mechanism of the gun or the rib extension, but no one in the trade could afford to spend this much time on every gun that needs to be color cased. That can also be proven by examining for example, a Turnbull, Menk, or Classic Guns recolor, and you will see colors surrounding every part of the action (most interestingly the breech face and water table, ie, no jigs or braces used there). Now I could be wrong and maybe they do spend hours making and modifying jigs for all of the different types of actions just so they can run at critical temp and hopefully prevent warping, but I doubt it simply because they would lose their asses on every job. So, one is left wondering how they achieve those results without sacrificing colors on those areas, and the answer is exactly that of which we all mentioned, lower temps. Just as you said Alex, many of the old frames out there that many of us shoot hundreds, if not thousands of rounds through a year were probably superficially "hardened".
Here is a Sterlingworth I experiment with. Not exactly the right colors for a sterlingworth yet, so Ill re run it. Three runs so far (not counting the initial run it had at the factory) and NO warping, acceptable hardness, and the only thing Im not satisfied with is the colors yet.
