I have been using a Remington 870 Super Magnum (I'm assuming this is in the "express" line you speak of) with a black synthetic stock and black metal for the last four waterfowl seasons. I have shot a lot of shells through it. I have carried it through a lot of marshland. It has never given any sort of hint at performing less than perfectly. My shotgun looks and functions like new. I take considerable pains to take care of the gun and keep it clean. After every hunting weekend I clean it and leave a light coating of oil on the metal surfaces, including the barrel and receiver which are coated with some sort of rough textured stuff (parkerizing? tefflon?). If the gun gets wet during a hunting outing, I will dry it, clean it, and oil it ASAP when I get to the house/hotel. But I don't baby it. I haven't fallen in the marsh with it, but I have splashed some water on it while walking into the marsh in the dark, splashing after downed ducks, etc. It comes with the territory of duck hunting that a shotgun is going to get wet sometimes.
I have read that some of these 870s seemed to be susceptible to rust. Maybe I haven't had this experience because of my frequent care and coating of surfaces with oil. Maybe the reports were in the context of hunting around salt water -- salt water is more corrosive and would tend to rust a gun more rapidly. Maybe the problem was identified by Remington and corrected. I did see one report that Remington had changed the surface coating -- maybe adopting tefflon coating -- to address this issue, but I could not corroborate this report.
I'm a completely sold 870 express user. I bought mine for about $290 with the express desire to have an inexpensive shotgun that I would not worry about getting wet when duck hunting -- a new activity for me at the time. I reasoned that such a cheap shotgun could readily be replaced if it got worn out or fouled by the harsh waterfowl hunting conditions. I still think that is a solid way to think about the gun. If it goes to hell in four years, buy another one for about $300. It has not been my experience, however, that the gun degrades rapidly and needs to be thrown away and replaced. As I say, my four year old gun look and functions like new. Take care of them and they will last plenty long enough.
