A nail-driving gun barrel assembly includes an axially extending barrel and an ejector. The barrel has a firing end portion having a cylindrical segment and a protruding segment that cooperates with the cylindrical segment to define a first cutout recess. The protruding segment is formed with a first groove. The ejector has a sleeve part and a projecting part which cooperates with the sleeve part to define a second cutout recess. The sleeve part is coaxially and slidably sleeved on the cylindrical segment. The projecting part is formed with a second groove. When the ejector is disposed at a first axial position, the first and second grooves mate each other.