There is a very long line of responsibility for weapons on a set, but ultimately it all comes down to two parties that are pretty much equal in power, the director and the armorer. Of the two, the director has a nominal bit more power because the armorer is supposed to be able to tell the director NO if something unnecessarily dangerous is asked for, but the director can fire (usually, depends on contracts, studio policy, etc) the armorer.
The armorer is supposed to rule over the use and handling of all weapons and generally does so with a gleeful tyrannical overbearing manner. Movie making is all about power and throwing your weight around constantly and in my limited experience it is absolutely ugly. Apparently, this director had an uncanny ability to throw his weight around in such a manner that he could not get (or would not get) a qualified armorer to work on the project and the woman he hired was not up to the task of throwing HER weight around and enforcing normal rules regarding weapons, the first of which was adopted unanimously by the Union immediately after Brandon Lee was killed on set with a live bullet. And that is THERE WILL NEVER BE A LIVE BULLET OF ANY CALIBER OR SIZE ON A MOVIE SET. This person had never been an armorer on a movie set before, though from what I understand had worked in props and perhaps under an armorer on at least one occasion. She was unqualified. The director should have known that and insisted on a qualified party be hired.
"Well, it's only my lucky .22 shell from when I was a kid hunting squirrels in Tennessee and we are filming a Civil War scene with muskets", says some lowly production assistant. "Nope. Walkout! ATTENTION ALL IATSE MEMBERS....WE WALK!", says the Union armorer and the entire union membership performs a walk-out and filming stops until such time as proper safety measures are taken. And not just the Armorer and their assistants, but lighting, costumes, camera, electrical, building trades, properties, and even the persons that feed everyone on the set, the craft services.
You will note that early in the breaking of this story that it was reported union members walked. They walked because of repeated safety violations AND this director has a history of safety violations and putting people in danger (now THAT is a real power play, he is literally saying he is a god and can put your life in danger at his will). Unfortunately, this tiny production company was independent, not one of the major studios that work with the union. There were not enough union members to stop production until such time as demands for safety measures were reinstated. Turns out they were right, huh?
Yes, I get that whole thing about the only person really responsible is the person with the gun in their hand. But the reality is that on movie sets they do not hire properly trained gun handlers as actors. It is the responsibility of the armorer to train that actor (and everyone that handles the gun) in basic gun safety and see to it that the actor (and everyone else in between the armorer and the actor) strictly adheres to those gun safety protocols. There are different rules on a movie set, the rules you were taught about safe gun handling do not apply. The first rule you should have had had hammered into your head or else beaten into your butt is that you never point a gun at anyone unless you mean to kill them, right? Well, under your rule there will be some very major changes to movies. No more Dirty Harry asking a guy if he counted the shots, was it five or was it six, and did he feel lucky? How good would that scene have been if Harry was pointing the weapon AWAY FROM THE BAD GUY and not right at the spot between his eyes?
In my opinion the responsibility lies 80% on the director and 20% on the armorer. The young woman working as armorer should have known she was out of her depth and was not capable of standing toe to toe with the director, demanding safety measures be followed to the letter. But everything that happens on a movie set ultimately is the responsibility of the director. Guns with live ammunition were fired on the set in the preceding days. The director did not shut down filming and investigate the situation. He should have shut down things and called the armorer on the carpet, demanding to know who brought live ammo on set and who fired the gun(s) and who knew about it. The persons bringing live ammo should have been fired, the persons shooting the guns possibly fired as well if not demoted and seriously reprimanded, and those that knew about it and did not report this should have their butts chewed such that sitting would be an agony for a week or more. And lastly, the director should have fired the armorer and hired someone properly qualified.
I have been discussing this at length with a friend that is a qualified armorer in the International Alliance of Theatrical Stage Employees. He started his career when he was hired to work on the Dances With Wolves production and just recently finished his 5th project with Kevin Costner, a prequel to the series Yellowstone. He educated me on the details of Union rules for armorers, and I was hired by him to be his assistant to help him with a German documentary (more like a b.s. fantasy) that wanted Native American archery shots.
I get it that a lot of people always want to hang it on the loose nut on the trigger. And it is true that the loose nut on the trigger is the weakest link, but look at it this way: you and your whole family are gathered in the living room with one person that you really do not know well.....would you allow one of your kids to hand the new acquaintance a weapon that you do not know whether or not is loaded? And what if someone in that room were to be shot unintentionally? How do you look the rest of the family in the eye after that? There's your answer to the question about responsibility.