RE: My New Stevens 200
they're all test-fired from the factory to assure performance. unless it was stored in a damp/cool environment for a long time, it's probably not rust but copper. just use a foaming cleaning agent and wipe it clean after 20 minutes.
happened to the outside AND inside of my barrel after three months. Keep a good layer of hopped no.9 or breakfree or some other oil on it after you're dont using it.