Can you cite a law that causes this exception? If there is no specific law that specifically excludes actors from being reasonably responsible of the firearm, then he will be treated the same way I would be treated if some expert told me "it's fake" and killed someone.
But he was acting responsibly. Laws and regulations vary by country, but the person responsible for the weapon and what’s done with it is the armourer. In exchange, the actors have to obey the instructions of the armourer, and if they don’t they are in serious trouble. So like I said, Baldwin’s fault was trusting someone who was not the person in charge of the weapon, which is of course not good. He was being responsible insofar as he was told it was safe, so he was free to pull the trigger as often as he wanted, point it at other actors, at the camera, whatever is necessary for the scene. How do you think it works when an actor plays Russian roulette on a movie? The armourer would be on their ass watching them like a hawk, and the second they are done, they would take it back and put it back in the safe in their van, but they are still expected to point it at their heads and pull the trigger.
Baldwin did what’s expected of an actor. He’s not expected to check anything, he’s given a weapon and told it’s safe to press the trigger or use it as a hammer. He should have received the weapon from the armourer, not from someone else. The moment it doesn’t come from the person responsible for it, whether it’s safe or not simply becomes hearsay. The AD and the armourer are both in serious trouble, though (especially the armourer, because ultimately they are the ones responsible for what happens with the weapon).
Likewise, when an electrician certifies an installation as safe, they are taking on a legal responsibility. If you ask your wife to make a cup of tea and she gets electrocuted when she plugs the kettle in, you won’t be in trouble, whoever certified it as safe will be.
Not everything is an actual law, some regulations are not laws per se, but they must be adhered to and are admissible in court.
That’s not to say he won’t get prosecuted (and sued, of course) but those in real trouble are the AD and armourer.