Severly corroded ammo
The green stuff you see is actually the brass being eaten away. In light cases it is just surface corrosion/tarnish and can be removed by hand polishing as shown above. Given enough time or added moisture, a crater will actually form in the brass. It can be seen under magnification, or felt with your fingernail. The case wall thickness is decreased in this crater. The brass metal also becomes more brittle in this spot. Again, given enough time it will eat a hole thru the case wall. Firing severly corroded ammo could blow a hole in the case wall causing damage to the weapon/shooter. If your ammo does not clean up easily using a light polishing, serious thought must be given to just pulling the bullets, and scraping the severly corroded brass pieces. dozier