A Colt Detective Special
The Colt Detective Special was a snub-nosed revolver used primarily on Earth by plain clothes detectives in the 20th century.
In the Dixon Hill series of holographic programs, McNary, a detective with the San Francisco Police Department, was armed with this weapon. (TNG: "The Big Goodbye")
In the Hotel Royale simulation, the bellboy owned a Colt Detective Special for an eventual confrontation with Mickey D over Rita. He hid this weapon in his waistband. (TNG: "The Royale")