According to the post war analysis done by the USAAF, the ball turret was one of the safer positions to be in. According to the records of the 303rd BG, here is the order of safest to least safe, based on % of men killed in that position:
Position % KIA
Pilot-Copilot 14.20
Radio 12.97
Engineer 12.37
Bombardier 12.06
Navigator 12.06
Tail Gunner 12.06
Ball Turret 11.76
Waist Gun 10.23