Just what I've been saying...
The firing pin blocks on any auto pistol are not always completely Stop or Go. (That is thinking brought on by living in the Digital Age.) Due to mechanical tolerance stack-up, there is going to be a "gray area" in between the 2 states. So instead of being a 50/50 proposition, it's going to be something more like 49/49, which leaves 2% error. It's that tiny error space that we are discussing.
This is usually not a problem. If the hammer or striker is allowed to start falling early, then while the hammer is still falling the bolt is allowed extra time to move into it's final battery position. Thus due to the inertias within the system, the bolt is securely closed before the firing pin can strike the primer, which then starts the powder to burn. There is time lag in the hammer fall, and there is more time lag in the ignition sequence.
But anything that upsets that delicate timing sequence, anything that consumes those few milli-seconds of time... like dirt in the chamber, or out-of-spec ammo having to be forced home... could allow the ammo to fire OOB.
The higher the rate of fire, the more imperfect your ammo, the more slop in the mechanical tolerance, the more lax in your cleaning standards, then the greater chance for trouble.
I'm not saying there wasn't undue wear present on his firing pin block. That's not it at all. What I'm saying is don't look so surprised! Due to the way the simpler mechanical interlock systems work, this can happen on almost ANY auto pistol/rifle.
