That doesn't sound like a headspace problem. I wonder if (1) the chamber might have been opened up to say 9.3X62 or if (2) the caliber might actually be 9X57.

If the former, the firing pin blow would have pushed the cartridge far enough forward in the chamber to allow the primer to blow out when the case obturated and clung to the chamber walls. If the latter, the combination of an oversize bullet and insufficient neck clearance would have increased pressure to the point that the primer pocket would have expanded and released the primer.

What did the fired case look like? Did you measure the base of the case and compare it with the diameter of an unfired round? Would the primer just drop back into the primer pocket, or was the primer pocket still at its original diameter?

I hope you were wearing shooting glasses.