Short version: Has anyone here ever heard of Nitrate staining of steel, stemming from non-corrosive NATO primers?
Long version: I recently had a PTR-91 rebuilt by a seemingly well-established custom shop (I'm being deliberately vague, as I don't want to make any accusations just yet). However, upon receiving the rifle back, several of the internal components had significant rust. Since the shipping box was damaged, I assumed that moisture must have entered the action. The shop owner agreed over the phone, being emphatic that was no corrosion prior to shipping my rifle. Per his request, I returned it, for a more thorough inspection. He is now claiming that what I see is a purely cosmetic phenomenon, called nitrate staining. This, he states, is caused by residue from non-corrosive NATO primers. There are some used pats in the gun (it's a long story), but at least two of them DID NOT have any corrosion/staining as of 400 rounds ago and three months ago. That being said, many of the last 400 were South Korean and German surplus rounds.
The rifle is being shipped back today, so I will get pictures some more pictures once it arrives.