The only time I`vee seen something like your photo, is when a case was fired in a chamber with a little oil in it. The oil was in the barrel, and had run back into the chamber when the rifle was standing up in the gun safe. Oil is trapped between the chamber walls and case, and boils from heat of ignition, causing gas, causing dent. The pic of the case shows what I think is soot in the dent.
I run clean patches on the end of my rod into each chamber before shooting now.
I`ve also had a thread from a cleaning patch left in a chamber, the brass came out with a perfectly formed image of the intruder in it.
