[Ron Wood]

It is a time consuming process. The rust blue solution is hand swabbed onto the gun avoiding the bright polished inside, whereas the hot salt blue process involves dipping the entire piece into the bluing solution thus everything gets blued. Moreover, the rust blue process requires repeated application of the bluing solution, with boiling and carding of the surface between each application of the solution, until the desired darkness of color is obtained. The hot salt blue process requires only a one time dip so it is many times faster (but not nearly as attractive to my eye).
Ron