It is almost impossible DNA could travel backwards through a door into the barrel of Beau Lamarre-Condon's police pistol, his lawyer has suggested.
The former senior constable contesting two murder charges has argued Luke Davies was killed in a violent struggle for his police pistol, not murdered with a shot through a bedroom door in February 2024.
His NSW Supreme Court trial has been told DNA was found within the barrel of the alleged murderer's police pistol, but it was not able to be tested for technical reasons.
Crime scene expert Greg Moon concluded evidence on Tuesday and disagreed with Lamarre-Condon's barrister about the likelihood of DNA being able to travel through a hole in a door.
"The possibility of biological material finding its way from a wound to the deceased through a hole in the door ... and into the barrel of a gun - the likelihood of that approaches zero," defence barrister John Stratton SC suggested.
"I disagree ... I believe it can still happen," Mr Moon said.
"I'm putting to you it's very improbable," Mr Stratton said.
"I disagree," Mr Moon replied.
The bullet hole allegedly caused by the shot that killed Mr Davies was only found in July, more than two years after the men died, the jury of 15 has been told.
The hole was less than one centimetre in diameter.
Mr Davies, 29, was killed moments after his new boyfriend Jesse Baird was shot by Lamarre-Condon in Mr Baird's terrace home in Paddington, Sydney.
Lamarre-Condon, 31, accepts he shot Mr Baird, who'd months earlier called time on a brief, second relationship.
But he argues he acted in self-defence after entering the home in disguise in February 2024 to retrieve personal items.
The Crown argues Lamarre-Condon murdered Mr Baird out of jealousy and then eliminated the witness, Mr Davies, as the 29-year-old tried to make a triple zero call.
Mr Moon also gave evidence about the phenomenon of stippling, where gunpowder particles ejected when a gun is fired cause a cone pattern on a surface depending on the distance from the firearm to the surface.
On cross-examination, Mr Moon said the only wound he saw with stippling was the gunshot wound to Mr Baird's right temple.
Mr Moon said biological material would not be found in the barrel of a gun if it was fired again, due to the high heat killing any DNA.
Both parties agree Mr Baird was killed first with two gunshot wounds before Mr Davies was killed with a third shot.
Forensic and ballistics evidence is expected to continue for most of the trial's fourth week before the jury is taken to view the Sydney terrace where the killings occurred on Friday.
The former officer has pleaded not guilty to both murders.
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