Corrosion Inhibition
There is more than one type of corrosion. Common corrosion is oxidation, a chemical reaction between ferrous metals and the oxygen contained in air and water. Rust is the best example for this type of corrosion. But there are also other types of corrosion, such as acidic corrosion, galvanic corrosion and stress corrosion. Salt water corrosion is a combination of oxidation and acidic and galvanic corrosion. It is virtually impossible to stop corrosion completely. Corrosion happens, albeit very slowly. It is, therefore, more honest to speak about inhibiting corrosion rather than stopping or preventing it.
It sounds to me like you had a chemical reaction. Something already on the gun reacted with it.
Lee