I have been carrying an issued M&P40C since April 2008 and even when exposed to rain I have not seen any rusting whatsoever. I have some co-workers that have had some pitting on their night sights but that is from failing to keep those clean and oiled. So far, the barrel and slide are spot free. I also have 3K+ rounds downrange in the M&P and the only noticeable wear is the chamber hood has some rub marks where it locks in and out of the slide when it cycles.

Needless-to-say I have been very impressed with the lack of wear on the pistol and that the melonite coated stainless seems impervious to rust from rain water. Although... I am not in an area that has salt spray or anything like that either.