I agree stainless steel is better able to hold off the rust process, but it is no guarantee of it. I personally think you were lucky and it probably does have something to do with your climate. That would have never happened where I live. That rifle would have been in a bad way. Your friend should be ashamed of himself.

I don't care how good a friend he was.. he'd never get the loan of another anything from me. A pet peeve of mine is when someone borrows something of mine and then does not take care of the item.If I borrow an item I return it in better condition (if possible) then when Iborrowed it.

Many years ago, I had a friend that we had a real falling out. He borrowed an expensive piece of equipment from me. I really did not want him to use it and explained that this item had to be cared for!! Or it would be junk. I told him the necessity of taking care of it after being used, and he never returned it at all. I finally went to his house and demanded it back. His excuse, it never worked so he threw it away for me. Well I called BS on that because when I took it over to him, I set it up, and showed him how to work it (a paint sprayer). I felt it should be replaced, he refused. So I told him never EVER ask me for anything else to borrow. He later, came over to try and borrow my safe way scaffolding. I laughed at him and walked off. And then we had some words. Well that was the end of that friendship. And you know what, I really don't care.