It is stain-less, not stain-none.
Chloride is one of the common things that can cause many grades of stainless to corrode.
Sweat or chlorate (corrosive) primers are a couple common cources.
It often appears as a white roughening on the surface of the staiuless.
Some hot water (get the gun as hot as the water to aid faster drying).
Keep in mind that some action parts may NOT be stainless (it makes lousy springs).