A movie set weapons handler who loaded a gun for actor Alec Baldwin before it fired and killed a cinematographer was "sloppy", her trial has heard.

"Negligent acts" by armourer Hannah Gutierrez-Reed, 26, led to Halyna Hutchins' death during the production of Rust, prosecutors told the court.

Ms Gutierrez-Reed has pleaded not guilty to charges of involuntary manslaughter and tampering with evidence over the fatality during the production of the Western in the US state of New Mexico.

During opening statements for the trial in Santa Fe on Thursday, prosecutors argued that the defendant's "unprofessional" on-set conduct led to an avoidable tragedy in which live ammunition found its way into a weapon.

"You will hear testimony that she routinely left guns and ammunition lying around the set unattended and her gun safe and ammo cart were constantly disorganised," special prosecutor Jason Lewis said.

To make their case, prosecutors showed jurors boxes of ammunition that were on set the day the shooting took place, which they said contained live rounds interspersed with dummy ones.

They allege that Ms Gutierrez-Reed negligently brought live ammunition from her home to the set that day, including the bullet that struck and killed Ms Hutchins.

The defence case is basically to just keep saying that Alec Baldwin is at fault because he fired the gun. I'd love to see him do prison time but the armourer sounds much worse.