I’m surprised no one has mentioned using a desiccant. Do you use any type of dehumidifier or desiccant in your safe? This will help control moisture.I own two blued Knights, an American and an MK-85. I bought both of these used. The exteriors are in good shape. On both, I always get rust on the patch after and in-between cleanings. I've tried everything I can think of, repeated cleanings, steel wool, and bore paste. I am using a quality oil to protect the bore. Anything else I should be trying. I've tried to live with it, but it bugs me. I haven't had these issues with my stainless Knights, once I got them cleaned up the patches come out white. Thanks as always.
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I have known BR shooters to coat the inside of their Bores with patches saturated in graphite. Lock eeze is a common product to use. This coats the inside of the bore and fills in the voids that can lead to throat erosion (or so the logic goes). Might help a lot with corrosion. The lock eeze can be applied wet on a patch, then left for the solvent to evaporate. A dry patch later will remove graphite, but a lot of guys just shoot it that way.
Good luck! Brown patches are no fun…