Another +1 regarding long term storage in a factory foam lined case causing rust and pitting. Had this happen to me with my first semi-auto, years ago, after I stopped going to the range regularly. I was even warned about this but didn't think anything of it, and it sneaked up and bit me. Best to store firearms in an environment where nothing that can hold moisture (like foam or carpeting) is touching the metal surfaces.
It's especially important to know that if you rarely shoot your guns, to religiously maintain them by giving them a lube rub every month or so. You'd be surprised how rusty they get just sitting around doing nothing when you don't maintain them!