Safety standards developed by film studios and labor unions are the primary protection for actors and film crews when a scene calls for using prop guns. The industry-wide guidance is clear: "Blanks can kill. Treat all firearms as if they are loaded."

Shootings nevertheless have killed and injured people while cameras rolled, including the cinematographer who died and the director who was wounded this week when no one realized a prop gun fired by actor Alec Baldwin during the filming of "Rust" carried live rounds that are far more dangerous than blanks.

Despite some industry reforms following p...