Manchester shopkeeper Pragaret Singh, 35, was shot on 31 October 2014 - three weeks after 26-year-old Kieran McGrath was killed in Ashton-Under-Lyne.
Detectives said tests showed the same pistol was used in both shootings.
Police do not believe the victims are connected, but it is thought the weapon may have been passed between criminals.
A Â£25,000 reward is being offered for information leading to successful convictions.
Five men have been arrested in connection with the shootings.
A 30-year-old man arrested on suspicion of murder and a 28-year-old man - held on suspicion of conspiracy to rob and conspiracy to murder - have both been released on bail.
Three more men - aged 32, 30 and 18 - have also been bailed after being arrested on suspicion of murder.
Mr Singh, also known as Charlie, was killed in a raid on Manchester Food Traders off Wood Street, Openshaw.
Tests confirmed the same .45 calibre self-loading pistol was fired in the murder of Mr McGrath in a "targeted" attack outside the Sheldon Arms pub in Ashton-Under-Lyne.
Det Ch Insp Richard Eales, of Greater Manchester Police, said: "I understand people may be afraid but we believe someone out there knows who is responsible and the information you supply will be treated in the strictest of confidence."
Crimewatch will be shown on BBC One at 21:00 GMT
Police investigating the murder of two men who were shot with the same gun will make an appeal for public help on the BBC's Crimewatch programme later.