I think 2-piper summed it up well. I suspect, Dtm, you are referring to actual heat treating and not coloring for appearance. I think torching to simulate case colors is frowned upon because its not a flattering gun finish appearance, but it may also spot anneal in unknown variations.

An important consideration could be that gun parts may not necessarily need a very high surface hardness. Case coloring for appearance may be the practical goal without getting into the worry of distortion for various reasons. If a part was truly flame hardened by the book, that property may be lost if some degree of heat is used to create colors thatre desired. Only thoughts is all.