Three things can happen; the turret ball can collapse, the turret mountings can detach and the ball end up in the fuselage or the aircraft structure can collapse. I'm sure there were a lot of belly landings without a trapped gunner.
This is a movie of Stunt Pilot Many Manz crashing a B17 for a movie. There is no Belly Turret.
This is a video of an actual 1944 B17 belly landing. The narrator says "ball turret jettisoned to save breaking his back"
This is a B17 crash landing with only one wheel, the turret is still present, the remaining undercarriage leg does not collapse
This is a B17 crash landing with only one wheel, the turret is still present, the remaining undercarriage leg dose collapse half way through the landing but there appears no damage or collapse. Interesting is the ground crew that come along with air bladders to inflate and lift the aircraft. Also what looks like a fireman in PPE with some kind of breathing bladder on his back.
I can not find video of a ball turret collapse.