The prop gun that Baldwin fired contained a “live single round”, according to an email sent by the International Alliance of Theatrical Stage Employees to its membership, reports Variety.
In Rust, Baldwin was starring as an outlaw whose grandson is sentenced to hang for an accidental murder.
The actor is best known for his role as Jack Donaghy on the NBC sitcom 30 Rock and for his portrayal of Donald Trump on the sketch show Saturday Night Live.
Such incidents on film sets are extremely rare.
Real firearms are often used in filming, and are loaded with blanks – cartridges that create a flash and a bang without discharging a projectile.
In 1993, Brandon Lee – the 28-year-old son of the late martial arts star Bruce Lee – died on set after being accidentally shot with a prop gun while filming a death scene for the film The Crow.
The gun mistakenly had a dummy round loaded in it.
Media caption,The director who worked with Halyna Hutchins on the 2020 action film Archenemy says her death is “unfathomable”
How prop guns work in films
According to the Los Angeles Times, about half a dozen members of the camera crew on Rust walked out hours before the tragedy after protesting over working conditions on the set at the Bonanza Creek Ranch near Santa Fe.
The union members had reportedly complained that they were promised hotel rooms in Santa Fe, but once filming of the Western began they were required to drive 50 miles (80km) from Albuquerque every morning.
Meanwhile, the BBC has obtained a document showing which crew members were listed as scheduled to be on set that day.
It names a head armourer – the crew member responsible for checking firearms – who is in her twenties and had recently served in that role for only the first time.
Prop Gun contained Live Single Round-Alliance of Theatrical Stage Employees