I've had it happen twice on the same rifle. First time was when it was brand new, I was on my second mag and it made a funny pop when I pulled the trigger. Upon on inspection I found the case in the same condition as yours.
It happened with Remington GB . I sent it back to S&W and they upgraded the springs and what ever else. It happened a second time a couple years after the first with Remington GB but this time it blew the extractor out. I ended up making an extractor for it and it's still humming along today.
The thing I noticed was that the bullets on the GB's would stick in the chamber and not allow the bolt to go to battery and this is what caused both my OOBD's. Now I always make sure I scrub the chamber often when using GB's or not use them at all. The bad thing is I have a **** load of them so I watch when loading and make sure I don't load any that a bit suspect
