The union's focusing on that aspect of the story because on it's own you're completely right - it's not an actor's job to be a gun safety expert.

The question before the court is whether Baldwin bears responsibility for creating an unsafe environment on set, ignoring the many safety complaints of the crew, ignoring the previous negligent discharge incidents that had occurred on set, ignoring the previous negligent discharge incidents on other sets that some of the crew had been involved with, hiring an incompetent armorer, not hiring the armorer to be an armorer for every day that real weapons were on set, and so on. I guess if the producer credit that Baldwin has is a "vanity credit" and he wasn't involved in any of those decisions as the tweets above describe then he has a case, but if he was legally responsible for the actions of the production company in some way then he bears a share of responsibility for some or all of those things.

AFAIK the assistant director took a plea deal and the armorer has a separate case, her defense relies on the fact that she was only hired to be a "prop master" and not an "armorer" on the day that the shooting occurred.