Alec Baldwin’s lawyers have argued damage to a revolver that killed a cinematographer on the set of Rust has stripped them of a proper defence at the actor’s trial and asked a judge to dismiss an involuntary manslaughter charge.

“They understood that this was potentially exculpatory evidence and they destroyed it anyway,” Baldwin lawyer John Bash said during a virtual court hearing.

“It’s outrageous and it requires dismissal.”

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

Judge Mary Marlowe Sommer said she expected to issue a ruling on the motion to dismiss on Friday (US time).

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

Sheriff’s investigators initially sent the revolver to the FBI solely for DNA testing. When an FBI analyst heard Baldwin say in an ABC TV interview that he never pulled the trigger, the agency told the local authorities they should conduct 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 but the authorities went ahead 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,” 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 Marlowe Sommer declined to throw out the case, she should at least not allow any of the technical gun analysis to be presented at trial.

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

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

Armorer Hannah Gutierrez-Reed was convicted in March of involuntary manslaughter for her role in the shooting and was sentenced to 18 months in prison.