According to this diagram that should not be possible since if the guns were pointed into the tail gunner's position they should not have been able to fire due to the "tabo" arms. However, if one of the "tabo" lever arms failed (or had been shot off)....who knows? I could imagine a gunner shooting at a plane attacking from the rear (6 oclock high) and diving down toward the plane. If the gunner was relying on the "tabo" mechanism to stop the guns and there was a failure....? Do you know any of the details/circumstances when this happened?
http://www.lancaster-archive.com/lan...per_turret.htm