I've seen guns in that condition come from fairly open livestock barns.
Maybe the product of urine in the air?
Soaking in Evap-O-Rust will change the color to a flat black on rusted surfaces, but if it'll get a total refinish what harm is there in that? It definitely will remove proud rust... and open pits you didn't realize were there. It will, however wreck any patina one might want to preserve on a project. Your mileage may vary.
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