Juice wrote:I am a BP shooter as well. Someone correct me if I am wrong, but BP fouling and the salts left behind from surplus ammo cause rust by absorbing moisture from the air. It's the resulting water that really does the damage...
Iron (steel) rusts by oxygen, not water per se. The water, if present, acts as a catalysis in the process.
Witness that a water system like a circulating heat transfer system, for example a commercial building's cooling towers on the roof, will have an iron impeller pump because it's cheaper. The sealed system has little opportunity for excess oxygen to get in, and any small amounts of dissolved O2 quickly get scavenged out by minuscule rusting.
But a fresh water system, or one that is exposed to air like a hot tub recirculating system uses a bronze impeller because the water will continuously dissolve new O2 into itself, and the iron impeller will rust away rapidly.
So the water itself doesn't rust the metal, the O2 rusts the metal. But the water accelerates the process.
Chlorides, however, will directly attack the metal. And the corrosive primers leave chloride salts in the bore. The phenomena in high pressure systems is called chloride stress corrosion. Obviously the rifle's bore isn't under any tensile stress sitting in a rack after a trip to the range, but the chlorides are still reducing the iron by chemical corrosion, with or without water present. Albeit far more slowly without water. So South Carolina's high humidity is worse than Arizona's low humidity. But you don't get a free ride in Arizona either. The chlorides are still there even in the dessert. They just don't get an assist from the humidity like they would in a southeastern state.
"The saving of our world from pending doom will come, not through the complacent adjustment of the conforming majority, but through the creative maladjustment of a nonconforming minority.” - Martin Luther King, Jr.
Once in a lifetime lucky, corrosive primers deposit rust in a bore that can start rusting in less than an hour under humid conditions, but black powder with saltpeter as a major component is far more corrosive to metals if not taken care of right away. I guess it was winning lotto ticket, or unrusted Sharps as your once in a lifetime lucky strike. 
