probably laying on the ground and got wet and corroded just on the part in the ground
The reason I have been told that this happens is because the load may not be hot enought to make the brass expand enought to seal all the way around the end or the case mouth. I tried it in a revolver by reducing the loads by 1/2 grains and my findings were that the lighter the load the worst this happens. My conclusion was that this was the most likely reason for the blacking of the case. Just my 2 cents worth.
Graydog





























