Note: Getting this achievement does not impede your game progress.
This achievement is automatically unlocked on the road to the "Void" achievement.
In the basement, you'll find a wall that looks newly plastered. The wall is a 45 degree diagonal to the rest of the walls. Interact with it to break through.
Go through the newly made opening, down the stairs, and into the familiar room. Click on the revolver. Press E to point it. Press E again to use it. The achievement will pop!