The gun used to kill father-of-three shopkeeper Charlie Singh in Openshaw last month was also used to murder Kieran McGrath in Ashton.
Ballistic tests have now shown that the gun used to shoot the two men was a .45 calibre self-loading pistol.
Detectives are now working to establish whether the weapon was moved on between criminals after Kieran’s shooting.
Kieran, 26, was gunned down outside a pub in Ashton on October 4, while only weeks later Charlie was killed while locking up his shop in Openshaw on October 31.
Detective Chief Inspector Richard Eales, who is leading the Charlie Singh investigation, said: “We have decided to go public with this update now as we believe it is important know that two innocent people have been killed with this one gun.
“It is obviously deeply concerning that one weapon has been used to take the lives of two men and it is natural that the affected communities will be very worried about it.”
Detective Chief Inspector Eales explained that it was not uncommon for police to discover a weapon to be used in one than one, sometimes unrelated crime.
“In our experience it is not uncommon for firearms to be passed round between criminals and used in more than one offence,” he said.
“As far as we are concerned there is no link whatsoever between Charlie and Kieran and the only connection between them at this stage is the weapon used.
“As you would expect we are also examining the possibility that there is a link with the offenders, but we are keeping an open mind.”
A 30-year-old man was arrested on suspicion of Charlie’s murder and has been released on bail until 28 January 2015.
Two men, aged 30 and 22, were arrested on suspicion of Kieran’s murder and have been released on bail.
“We have made a number of arrests in connection with these murders, including one man, aged 30, for both incidents. However, I would like to stress that, right now, the only definite connection appears to be the firearm used,” Detective Chief Inspector Eales added.
Anyone with information is asked to call police 101 or Crimestoppers on 0800 555 111.