That’s what I’m trying to wrap my head around. How did the firing pin strike that deep without the bolt in battery? All the evidence I can see ( with my limited knowledge of the platform) says it was not.
What I see in my mind, without a BCG here to play with, is the powder igniting, sending the case back into the bolt face. The cam pin is not contained so the bolt rotates (cam pin scarring receiver in the process) allowing more pin protrusion. When the BCG reaches the rear of the receiver, the inertia keeps the carrier moving but the bolt is stopped, forcing it to rotate to its normal position with the firing pin retracted as evidenced by the picture. If the BCG was removable you would see more damage to the upper receiver due to the cam pin being forced to rotate outside the race. Am I dreaming in technicolor?