John Paul Evans, 57, and his son Keith Evans, 31, are accused of murdering Jesse Thompson during a pursuit through the streets of Wyong on the NSW Central Coast in 2017.
The pair were in a dual-cab ute with a woman, who cannot be named for legal reasons, when a shotgun was discharged in the hands of Keith Evans.
Mr Thompson was sitting in the back seat of a Toyota Kluger, the second car involved in the pursuit, when he was hit by the shot.
The 19-year-old died soon after as a result of his injuries.
Events preceding Mr Thompson's death were revealed as agreed facts during the NSW Supreme Court trial opening on Thursday.
In the days leading up to his death, Keith Evans assaulted one of Mr Thompson's friends, Jayke Rodgers, putting him in hospital.
On the day of the alleged murder, Mr Rodgers and some of his friends retaliated by smashing a glass door at the Evans' family home.
They later met with more friends at a local park and got into the Kluger, with Mr Thompson in the back seat, and became involved in a car pursuit with the Evans' ute.
But the details of the car chase, including who brought the gun, remains in dispute.
One of the key issues in the trial is whether Keith Evans intended to fire the shotgun.
"The heart of this dispute is about the discharge of that firearm and whether it was accidental," his barrister Michelle Fernando said.
Her client did not intend to fire the gun or harm Mr Thompson, Ms Fernando told the jury.
"Keith Evans is here not because he's committed a crime," she said.
"He's here because the Crown alleges he has committed the crime of murder."
John Evans' barrister, John Stratton SC, told the jury his client was not aware of the altercation between his son and Mr Rodger's group of friends.
"It is his belief that the shot which killed the deceased was the result of a terrible accident," he said.
Mr Stratton said John Evans maintained he picked up the gun after one of the men from the Kluger dropped the weapon.
Crown prosecutor Adrian Robertson told the jury John Evans has given evidence he knew about the smashed door at their family home.
Throughout the trial, occupants of the Kluger would tell the jury they saw Keith Evans point the gun at their vehicle, Mr Roberston said.
The jury would hear evidence Keith Evans first pointed the shotgun at the front of the vehicle, before shifting it to fire through the open back window, he said.
Mr Robertson said the Crown must prove he deliberately fired at Mr Thompson.
"An intentional act, rather than an accidental one," he said.
"And of course, that there was no lawful excuse for such a thing."
The trial continues on Friday.
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