I get that too, out of my 12 BVSS in 223. It appears as small L shaped dent, like the corder of a small square tool being pressed into the brass. I suspect it's a chip imbedded in the chamber left over from chamber ream. I can identify brass from my gun by the mark it leaves right below the shoulder.

I just returned a new 12 FV because of a rough chamber. It was tearing up the brass upon eject. Tried wire brushing it with chamber brush, but it didn't improve it a bit.