Josh Taylor, 19, remains in a critical but stable condition in Westmead Hospital after he suffered a single bullet wound to the side of his head in Pitt Town, in Sydney's north-west, about 6.30pm on Friday.

Mr Taylor was sitting in a car outside the IGA supermarket on Eldon Street with three friends, who told police he took a gun, held it to his head and fired while playing the deadly game of chance.

The only bullet in the gun discharged and entered his head, several centimetres above his ear.

Mr Taylor's sister learnt her brother had been shot when she logged on to Facebook and saw distressing posts about the incident.

Hysterical, she then phoned her mother to deliver the shocking news, Fairfax Media understands. That was before police even had the chance to contact them.

Detective Inspector Garry Sims said Mr Taylor's family were keeping vigil at his bedside in hospital, where Mr Taylor was believed to be on a ventilator.

"He is surrounded by his family, and obviously they're just waiting to see how things progress," Detective Inspector Sims said.

"I've been in contact with his mother. She's obviously beside herself. We all just express our feelings for the family."

Detective Inspector Sims urged people to be respectful about what they posted on social media following the incident.

"There have been a lot of comments that have been harmful and hurtful to the family," he said.

"We ask people to consider the fact that there is a family with a great deal of stress at the moment. The key message police are asking is for people to show restraint, respect and courtesy."

On Sunday afternoon, Mr Taylor's mother posted on Facebook: "Come on mate, fight!!! WE need you home with us ... no words can comprehend."

One police officer said, in a "fluke of ballistics", it was possible to survive being shot in the head if a bullet deflected off the skull. It was unclear whether this was what happened in Mr Taylor's case.

Detective Inspector Sims said police had recovered a gun from the vehicle and it was being forensically examined. Police were making inquires into the origin of the firearm, he said.

In November last year, Mr Taylor had uploaded a photograph of a handgun to his Facebook page.

"Thats redhot," one of his friends commented on the photograph, one of several on Mr Taylor's page that features guns.

"So is fire," responded Mr Taylor.

Detective Inspector Sims said, in his 30 years in country and metropolitan policing, he had never investigated a suspected case of Russian roulette.

"You see these things on movies and things, but I've never experienced anyone who would consider playing Russian roulette with a loaded revolver," he said.

Investigations into the shooting are continuing, and police have urged anyone with information to contact Crime Stoppers on 1800 333 000.

There was no evidence that anyone else in the car was involved in the game apart from Mr Taylor, Detective Inspector Sims said.