The Double-Action Revolver is a hidden weapon which unlocks after completion of the special quest. It was added to the game as part of The Doomsday Heist and became available on December 15, 2017.
To start the Treasure Hunt in GTA Online, you have to join an online session and play in Freemode. In five minutes you’ll receive an email with a picture of a specified location.
Head to the marked area and find the note. You will hear a chime when you’ll approach it. The closer you are, the louder a chime. The note can be in one of the twenty locations shown in the screenshots below.
When you’ll find the note, you’ll have to examine three clues. Each of the clues is located in a fixed place, you need to investigate all three in any order.
Empty Gun Case
When all the clues will be found, the icon of the chest will appear on the map — this is the final location of the quest. Go there and take the revolver.
Double-Action Revolver Headshot Challenge
When you acquire the weapon, you can complete Double-Action Revolver Headshot Challenge. Get 50 headshots by killing other players or NPCs to receive a reward of GTA$ 250,000 and an exclusive variation of this revolver in Red Dead Redemption 2 (you’ll have to login with your linked Social Club account).
Video walkthrough: youtu.be/Tlo0jiGJGXI.