Just getting around to loading some 10mm Starline/Midway brass I've had a few years
Quite a few of them have a rust/dark stain in the primer pocket, and a few inside the case near the flash hole.

Anyone run into this before?
I'm tumbling them now, we'll see if it cleans up.

Here's a pic

It is simply tarnish as a result of the final washing. It is cosmetic, put a primer in it and you will never know which which ones they were. This is common to all brands of brass which are not Nickel plated.

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Gunsmithing since 1961