As someone posted earlier on this thread, it is almost always a series of events/errors that end in tragedy. This has to be the case here as well, but with a serious question on where or how the grievous round entered the set. Someone brought this round into play either intentionally or haphazardly. Either way that person is at the very least complicit in this death.

Next error is by the armorer that missed or did not ensure that a live round was not in the weapon. Or did not have enough knowledge or training to understand the identity of a live round or the appropriate insert for the purpose.

Third would be the armorer not controlling the weapon up to hand off to the actor. The Assistant Director should not be a middle man in this exchange. I would think the armorer would open the firearm, pull out the round and show the actor exactly what was going into the firearm and give last minute instruction before transferring possession.

Fourth would be the actor pointing a firearm with anything in the chamber at anyone and especially for a camera alignment when it was unnecessary for anything to be in the chamber. The L.A Times article states that the camera operator and director and assistant watch the actual filming from a safe room when a blank may be necessary for the take.

Fifth would be the director allowing a firearm with anything in the chamber to be used for a camera alignment. No need and an unnecessary risk.

Sixth would be the actor not taking firearm handling seriously. Pulling the trigger would be completely unnecessary for camera alignment. Especially with people manning the camera at the time. This supposedly veteran actor did not have his head in his job.

Who is responsible? All of the above. All are deficient in professionalism and seriousness of performance. None should ever work in this capacity again as a signal to everyone in the industry to take your job seriously.