After four weeks and more than 50 witnesses, the prosecution wrapped up its case Monday in the murder trial of Napa dentist Michael Posey. It was then the defense team’s turn to put on their witnesses.

Defense attorneys Colin and Kellin Cooper’s first witness contradicted the four expert forensic witnesses who testified the gun that killed Michael Posey’s wife, Elizabeth Posey, jammed at one point during the incident that resulted in her death.

Michael Posey, 54, is on trial for the shooting of Elizabeth Posey, 27, on April 19, 1996. Michael Posey told an investigator on the day of the shooting that Elizabeth Posey’s death was a “tragic accident.” In that interview and a 2005 interrogation after he was arrested, he said Elizabeth Posey accidentally shot herself during a struggle for the gun after she shot at him during a domestic dispute.

The couple had two boys, then 1 and 3.

Since the trial started on Sept. 19, the prosecution has tried to convince the jury of six men and six women that the Walther PPK .380-caliber gun jammed during the altercation. But when police came to the home on Maplewood Avenue in north Napa, the gun was not jammed.

That would suggest Michael Posey repaired the jam after Elizabeth Posey died and placed the weapon where police later found it, near her hand.

On Monday, John Jacobson, a forensic scientist, testified that in his view the gun did not stovepipe — as attorneys in the case describe the malfunction — or jam.

“I fired the gun 70 times, and not one time did it jam unless I intentionally tried to stovepipe it,” Jacobson said.

Jacobson also disagreed with  prosecution experts regarding the significance of a dent on marks on the casing of the bullet that killed Elizabeth Posey.

“The mark on the (casing) was created when it ejected normally from the firearm,” Jacobson said.

Prosecutor Rob Wade asked Jacobson to explain differences between his findings and those of the other experts.

Jacobson testified that prosecution experts never compared the condition of the two bullets that were found at the scene of the crime, but only tested separately whether the gun was likely to jam.

Wade elicited testimony that Jacobson never test-fired the weapon while holding it up vertically — the position it was believed to be in when Elizabeth Posey was shot — while other experts did.

When Jacobson finished, the defense called Berkeley clinical psychologist Dr. Daniel Goldstine. Goldstine, who said he has testified in more than 50 trials as an expert witness on matters such as post-traumatic stress and the state of mind of people under duress, was asked to explain Michael Posey’s often vague answers during his two interviews with Napa County District Attorney’s Office Investigator Ed Knutsen.

Defense attorney Kellin Cooper asked Goldstine, “What happens to the body and mind when you witness something traumatic like the fatal shooting of the mother of your boys?”

Goldstine testified that he believed Michael Posey had acute stress disorder — was in shock, in other words — during the 1996 interview.

Cooper asked if it would be unusual, considering that Michael Posey had just witnessed the death of his wife, for him not recall details such as where the gun was or in what direction Elizabeth Posey’s body fell.

“Not at all, sir,” said Goldstine.

If Posey was in shock for the first interview, would there be any reason to believe he could remember the details in an interview nine years later, Cooper asked?

“No. I would be astounded if anyone could answer those kinds of questions nine years later,” said Goldstine.

Under cross-examination from Wade, Goldstine acknowledged that some of the psychological impacts on a person who witnesses a traumatic event might surface even if the person was directly involved, and that it was “unusual” that Posey answered some questions in the 1996 interview without problem but was vague on others.

Goldstine also noted that Posey cooperated with police and offered to take a polygraph test. “This is a man who understands (the idea) ‘ye shall know the truth and the truth will make you free.’”

Goldstine also described Michael Posey as a “strange bird.” At one point, in an effort to describe the changing phases of a person recently shocked by a traumatic event, Goldstine hummed the theme music of “The Twilight Zone.”

Attorneys in the case say they expect it to wrap and go to the jury for deliberations this week.