Alec Baldwin’s lawyers have argued that damage done during FBI testing on a revolver that killed a cinematographer on the set of the film Rust has stripped them of the ability to mount a proper defence at the actor’s forthcoming trial, and asked a New Mexico judge to dismiss the involuntary manslaughter charge against him.

“They understood that this was potentially exculpatory evidence and they destroyed it anyway,” lawyer John Bash said during a virtual court hearing on Monday. “It’s outrageous and it requires dismissal.”

Prosecutors argued that the gun breaking into pieces during testing was “unfortunate” but that Baldwin’s team still has plenty of evidence for a defence and did not meet their burden for having the case thrown out.

Judge Mary Marlowe Sommer said she expects to issue a ruling on the motion to dismiss on Friday.

During the fatal rehearsal for the Western on October 21 2021, Baldwin was pointing the gun at Halyna Hutchins on a film-set ranch when it went off, killing her and injuring director Joel Souza, who survived.

Sheriff’s investigators initially sent the revolver to the FBI only for DNA testing, but when an FBI analyst heard Baldwin say in an ABC TV interview in December that he never pulled the trigger, the agency told the local authorities they could carry out an accidental discharge test.

The FBI was told to go ahead, and tested the revolver by striking it from several angles with a rawhide mallet. One of those strikes caused the gun to break into three pieces.

The FBI had made police and prosecutors aware that the test could do major damage to the gun, which had not been tested by the defence, but the authorities went ahead with the test without bothering to disassemble it and photograph its parts first, thus eliminating their most critical evidence in the case, Baldwin’s lawyers argued.

“We can never use our own expert to examine that firearm,” Mr Bash said.

The prosecution argued that the gun was not destroyed as the defence said.

“The parts are still available,” special prosecutor Erlinda Johnson said. “The fact that this gun was unfortunately damaged does not deprive the defendant of ability to question the evidence.”

But Baldwin’s lawyers said the damage done to the top notch on the revolver’s hammer rendered the most important testing impossible.

They argued that if Judge Marlowe Sommer declines to throw out the case, she should at least not allow any of the technical gun analysis to be presented at trial.

Baldwin’s lawyers gave long and probing cross-examinations to the lead detective, an FBI forensic firearm investigator and the prosecution’s independent gun expert in evidence that is likely to be a dress rehearsal for the high-profile trial, where Baldwin, who was not on the online hearing, will be appearing in person.

The special prosecutors running the case argued that those cross-examinations proved that the defence has plenty of gun evidence to work with at the trial.

“They have other reasonable available means to making their point,” Ms Johnson said.

She added that all available evidence, from witness testimony to video of Baldwin firing the gun in film footage, showed the revolver was in good working order on the day of the shooting, and that police had no reason to believe its internal workings could provide exonerating evidence.

Prosecutors plan to present evidence at trial which they say shows the firearm “could not have fired absent a pull of the trigger” and was working properly before the shooting.

Baldwin Set Shooting
An aerial photo of the Bonanza Creek Ranch in Santa Fe, New Mexico, where Halyna Hutchins was shot dead during filming of the Western Rust (Jae C Hong/AP)

Defence lawyers are highlighting previously undisclosed expert analysis that outlines uncertainty about the origin of toolmarks on the gun’s firing mechanism.

Baldwin has pleaded not guilty to the involuntary manslaughter charge, which carries a maximum sentence of 18 months in prison.

Armourer Hannah Gutierrez-Reed was convicted in March of involuntary manslaughter over her role in the shooting and was jailed for 18 months.

On Friday, the judge denied prosecutors’ request to use immunity to compel evidence from Gutierrez-Reed at Baldwin’s trial. Her statements to investigators and workplace safety regulators are likely to feature prominently in the actor’s trial.

Last year, special prosecutors dismissed an involuntary manslaughter charge against Baldwin, saying they were informed the gun might have been modified before the shooting and malfunctioned. But they later received a new analysis of the gun and successfully pursued a grand jury indictment.