Omar Elomar, a relative of dead terrorist Mohamed Elomar, died after being shot in the chest at close-range at Miller Park, in Miller on February 15, 2020.
The 18-year-old had been sitting in a car with his younger cousin as members of the Elomar family and Isaako family gathered.
Noah Albert, who was connected to the Isaako family and arrived at the scene with the gunman, was acquitted of Mr Elomar's murder in the NSW Supreme Court on Friday.
The Crown had relied wholly on circumstantial evidence to support its contention that Mr Albert was in a joint criminal enterprise with the gunman to seriously harm someone.
But Justice Ian Harrison wasn't satisfied beyond reasonable doubt that Mr Albert foresaw the possibility that one of the men may be killed by the gun.
Mr Albert had denied he knew the gunman had a shotgun at any time prior to the shooting.
"The court ultimately determined that Mr Albert's account could not be shown to be untruthful or inherently improbable and therefore may possibly be true," a summary of the court's decision said.
The murder was the culmination of tensions between the two families over the previous 48 hours.
A teen boy from each family had fought on February 14, leading to a brawl involving 10 teenagers behind Casula Mall.
The court said older Elomar teenage boys then discussed the assault at a family function on February 15, leading one to exchange insults and aggressive messages with Mr Albert.
Ultimately they organised to meet opposite a school in Miller. The Elomars purchased a baseball bat and cricket stumps and distributed it among their group.
But within two minutes of arriving, the gunman who'd arrived in a car with Mr Albert approached Omar Elomar and shot him dead.
After the shooting, the accused allegedly falsely claimed to be the shooter.
"I think I killed the c*** delete everything I'll call you soon," read one message
And 20 minutes after that: "I met up with him and shot that kid in the chest".
The Crown said if he had no knowledge of a gun he would have reacted with horror, rather than falsely claiming credit for the deed.
Defence lawyer Paul Johnson argued his client had been trying to "defuse the impending confrontation" after the brawl, as seen in other text exchanges.