Re: What caused my case to do this? (Help)
It appears to be a classic situation of excessive case
lube (RCBS specifically), when resizing one's brass.
One of the many reasons I quit using this type of
sizing lube decades ago! For the last ten years or
so, I've been using sizing wax, which works perfectly
and cleans up easily.
I've never seen this as an aftermath of being fired:
being chambered looking perfect and coming out of
the chamber looking like this.
But since you didn't specify when this happened, and,
just in case it showed up after being fired, I'd suggest
it would be a result of anything caught in the chamber
between the loaded cartridge and the barrel's chamber.
Not having anywhere to go, it stayed put and dented the