An end piece or barrel head (1) for a rail vehicle axial slack adjuster is at its outer periphery connected to a barrel (2) of the adjuster and is arranged around a rotatably and axially movable tube (3). The end piece is provided with an internal, circumferential groove (4), connected to the exterior by means of a number of channels (5) opening in the end wall (1') of the end piece for allowing water collected in the groove to freely pass out to the exterior. The end piece is extended further into the barrel by means of a sleeve (1"), which is connected to the groove by means of notches (6) and contains a sealing ring (7) engaging the tube and sealingly cooperating with the sleeve.