Looking into some 40 S&W cases I see a circle at the very bottom, just over 3x the diameter of the primer flash hole. (Probably right on top of the smallest outer diameter part.) Is this a crack unto case head separation?
I'm not sure what you are describing. You can look into the bottom of a 40s&w case & see a possible case head sep, not possible w/ bottle neck rifle rounds. The "crack" line is about 2-3mm up from the bottom of the case head. It will appear as a bright line on the outside & a noticeable cut line on the inside.