One would think that soot would almost just rinse off, no expert here but I wonder if the fire with all the different chemicals in the smoke left something else on the metal. All kinds of compounds get created in fires and maybe you are dealing with a bit of a different problem. As far as how to get it off, if the standard gun products won't do it I'd get on line and try to find some help from fire damage sites. Being in the chemical industry I do know that carbons when heated will form some bonding materials which wiill stick to the first cooler surface they come in contact with ie. gun metal. I would remove the metal parts of the guns when using alot of wet products on them. Fires are a problem when all the plastics burn give off all kinds of bad things. Good luck, glad that some things were saved from the fire.