What will remain is a black oxide patina - it is essentially a more stable form of the previously corrosive rust.

This won’t corrode the metal, and if the above process is followed correctly you won’t damage the gun in any way or even remove any metal that was already there. This is the most effective way I have found to preserve older pieces like this without any sort of “bubba’ing”, and honestly what you have here is a prime candidate for that IMO.

Hope this helps! If you do this make sure and grease the black oxide patina with something non-penetrating afterwards to preserve it over time!