You can rust blue the tubes first.
Then carefully polish (repolish) the damascus breech section, the monobloc and rust brown that portion of the bbls.
Rust Brown requires no boiling water,,just repeated rustings and when doing a Damascus finish the rust brown finish must be 'etched' betw cycles.
Some finishers only etch every other rusting cycle, but no matter. The already Rust Blued portion of the bbls must be protected from the etching soln or it will remove the rust bluing as well/
One easy way to do that is to coat the blued section(s) with common shellac. A heavy coat and do the bores as well to protect them from etching.
Paint it on carefully at the edges of the sleeve joint. You will have a perfect line of brown & blue.
Keep a sharp eye on that edging after carding as it may need touch up if the carding starts to erode the sharp edge of the line betw brown and bluing.
Since the Rust Brown portions never go into boiling water (that'd turn them to Rust Blue),,the shellac will perfectly protect the blued surfaces and the bores from the etching soln.
Don't forget to protect the bores!
It does the job when rustbluing and then etching for a Black& White damascus pattern,,though I often recoated the bores after every 7 cycles of boiling and etching just to be safe.
