I have a remington 870 with the black metal coating and synthetic stock, it is giving me a lot more problems then any other gun i own. It gets a rusty film on the surface of the reciever. it isn't pitting or rusting it just seems like the black coating from the factory isn't as good as a blued surface to prevent rust.
If I clean it and oil it the rust film goes away for a little while but I'm looking for a better solution to protect it.