Authorities are investigating after paramedics rushed the boy, 11, to hospital just after 5pm on Monday from a home in Sydney's Hills District.
The boy remained in a children's hospital with his mother on Tuesday, awaiting surgery to remove the bullet.
Police were met by the victim, four 18-year-old men and a 44-year-old woman at the unit in Norwest on Monday.
Three men were arrested before one was charged with concealing a serious indictable offence and two were released.
Several witnesses are believed to have been at the unit prior to police arrival, and Superintendent Naomie Moore said detectives know who some are.
''A number of individuals that were inside the unit at the time are well known to NSW Police and their association with organised crime is being considered,'' the Hills District Commander said on Tuesday afternoon.
''We do not know, and we are not eliminating any possibility about how this tragic situation happened.''
Police previously knew of the address and had previously ''engaged with'' the occupants, she added, but all those at the address appeared to have been invited.
Investigations have so far failed to turn up a weapon.
In response to questioning, Supt Moore said she would not suggest there was a ''gunman, so to speak'' that police were currently looking for.
The nature of the gathering in the unit remains a line of police inquiry.
Supt Moore said police had yet to speak to boy's mother, who she previously described as distressed by the incident
The two men arrested and released may still face further police scrutiny.
Some of the known witnesses were now uncooperative, she added, confirming similar statements from state Premier Chris Minns.
The man charged with concealing an indictable offence, Isaac Bechara, faced Blacktown Local Court on Tuesday morning.
He did not apply for bail and will remain behind bars until his case returns to Parramatta Local Court on October 1.
Mr Minns said he understood the adults who were in the house were not co-operating with police and called for anyone with information about the incident to come forward.
The shooting was a tragic reminder of the danger of guns, he said.
"Often the people who are the victims of guns are innocent people. Many times, they're children," he told ABC TV.
Opposition Leader Kellie Sloane defended her party's vow to freeze a major gun buy-back in an interview with Radio 2GB.
She said criminals and terrorists would not ''walk into a cop shop and hand over their illegal weapons''.
Police established a crime scene at the Fairway Drive address in Norwest - an upmarket apartment complex - and an investigation is continuing.
Anyone with information has been urged to contact Crime Stoppers.
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