Blueing a barrel

[Tom]

Hi
I have been reading how if you were to hot blue a shotgun barrel it will fall apart.
Most of what I have read says the heat will melt the solder. Only one article mentioned the acidic solution will eat away the solder, either way I'm guessing at the end of the day you have no solder.
So my question would be how is a Vulcan steel set of barrels reblued ?
Is the only method rust blueing?

I have rust blued two rifles and they came out beautiful , but there is nothing blue about them they are black.
I have looked at many reblued Parker's and the color seems to be a nice dark blue.
Could anyone shed some light on what process and chemical is being used to create this dark blue Finnish.
Thank you, Tom