Barrel for a self-winding timepiece including a drum of axis AA provided with an internal lateral wall having friction surfaces alternating with locking structures forming salient or inward edges at the boundary of the friction surfaces and a mainspring forming a winding including an outer coil, which is friction coupled to said internal lateral wall and free to slip against said internal lateral wall in the event of overwinding of the mainspring, characterized in that said friction surfaces are demarcated, downstream relative to the direction in which said outer coil slips, by a rounded salient edge or an inward edge.