Hi Oskar
I have had a look in an old book I have which lists problems with gun barrels and likely cures some sound very strange to us modern folk but the ones I have tried in the past do work, there origins are mostly from the Brit Victorian era.
I found this which is listed as “Patch rusting in gun barrels” this can be treated by soaking a cotton rag with Phosphoric Acid and swabbing out the bore after cleaning and removing any residual oil. I have not used this method because I have never had the problem but it seems it may work and worth a try.
