Abstract: A barrel (1) for self-winding timepiece including a drum (11) provided with an inner lateral wall (12) and a mainspring (2) forming a winding including an outer coil (21) the end (22) of which is friction coupled with the inner lateral wall and free to slip against the inner lateral wall in the event of overtension of the mainspring (2). The end of the outer coil includes an external face (23) having two friction surfaces (24, 25) disposed in the top and bottom portion of the outer coil and arranged to be in contact with the inner lateral wall in such a way that the portion between the friction surfaces, referred to as central portion (26), is not in contact with the inner lateral wall, the central portion forming a reservoir (27) for a lubricant between the two friction surfaces.