Originally Posted by
Kiwi Greg
If the brass isn't hard onto the bolt face, when the firing pin hits the primer it pushes the case forward as far as the extractor or chamber will allow.
Then the powder ignites & the shoulder pushes forward & grips the chamber then the case head finds it has nothing behind it so slams back against the bolt, usually the primer has also backed out against bolt, then gets completely flattened looking a lot like an overpressure situation.
This sort of thing also happens when brass is too short for a chamber because of over sizing or sloppy Gunsmithing with long head spaced chambers.
This stretches the brass usually right where case wall meets the web.