Any random actor can't be expected to be knowledgeable enough to assure a gun is incapable of causing injury. It's just not why they hire them, even if us gun people think the four rules should be written into the federal code. So the responsibility must fall to the person/s who run the show. I don't know if this is person is the director, producer, or some other title, but it's the same person who would be held responsible if, for instance, they hired an unlicensed cousin to fly a helicopter that crashed into the set. If they hired a competent armorer, had a reasonable plan to eliminate mistakes, but the armorer made a one time mistake and mixed up a live round in with the blanks, the negligence is on him/her. But it sounds as if they went low budget and hired somebody who was grossly lacking in some way. Combine that with the previous negligent discharges (I too don't agree with the term misfire) and it seems criminal that they continued using that person.
All that said, my guess is they will consider it an embarrassing mistake of the people making the movie and call it a workplace accident. No charges, but they will have to pay millions in the inevitable lawsuits.