Police are hunting a gunman who shot dead his friend after a heated row in Sydney's west.


Police said they believe they know who shot Joshua George in the chest.


Detective Chief Inspector Mark Brett, Macquarie Fields Crime Manager said there had been an argument between Mr George and friends on Saturday and an arrangement to meet up on sunday.


Emergency services were called to Gidley Crescent about 1.30pm Sunday where they found Mr George with gunshot wounds.


"We believe that (argument) had some bearing on this incident today,"he said.


"We are seeking assistance from two males and a female seen with the victim before hand," he said.


Mr Brett said Mr George was known to police.


A nearby resident, Mary-Anne, told Fairfax she performed CPR on Mr George.


"We came out when we heard a gun shot and a women screaming,"she said.


"I did my best then an ambulance came but we later learned he died in hospital," she said.


Police said Mr George was taken to Campbelltown Hospital where he was pronounced dead.


A crime scene has been set up at the scene of the shooting and specialist officers are at the scene, where they are expected to stay for some time, police said.