How does that happen, given that there is a compressed spring preventing the pin coming back much beyond "full cock"? Did the base of the bolt shear off, or the collar on the firing pin?
I've had dozens of pierced primers, and always found that the primer gas eventually escapes around the side of the pin (normally blowing gungy oil out of the bolt body!) as soon as the cocking piece reaches its limit of travel. There doesn't seem to be enough energy in a primer to cause anything outside the normal working tolerance of the bolt mechanism.