450_366:

First of all, a double that has seriously come off face is damaged goods to begin with. Any metal that is exposed to stresses beyond its intended design is from that point on weaker then before. A rifle that has survived serious excessive pressure close to breaking point will not do so the next time.

So now you have a system that is already considerably weaker then before. A gun smith cannot 'know or see' the proper temperature for a particular gun if he does not know the exact properties of the metal in question. Depending on prior stresses he might not even be aware of, the carbon content and the percentage and quality of impurities of the steel in question, he might change the structure of the steel to something downright life-threatening. It might not blow up with the next regular load, but you are in danger territory a lot earlier. Just a regular propane handtorch can reach temperatures well beyond what is needed to either over-soften the metal or to turn it into brittle scrap metal. Guess what these guys use to heat up the massive locking lugs. And heat travels, especially quickly in thin barrels. This practice is not limited to doubles, by the way. It's a dirty secret among European gun smiths that they often use a blow-torch to heat up jammed barrel / receiver junctions in old Mauser 98 rifles when they want to use the parts of the old 98 for something else. Guns based on these once overheated parts tend to blow up after one or two decades normal use, when the culprit of the cause is long forgotten.