God made the rain <img src="http://jsramsbottom.co.uk/bbs/e/icon_smile.gif" alt="Smile" width="15" height="15"><!--graemlin:--> but if you don't clean them or at least ensure they are dried thoroughly after getting soaked, they will rust.
This usually happens in the invisable places like between the woodwork & the steel areas. A good drying & a bit of Browning Legia spray should keep it in good working order.
Some guns may need their stock removed to ensure proper drying & cleaning. Your gun your choice.