The invention relates to a barrel spring (2) including: an inner end (5) and an outer end; a first portion (50) having a first height (H); a second portion (52) having a second height (h) that is smaller than the first height (H) and being located near the inner end (5); and, at the second portion, e.g. at the inner end, a first attachment element (51) suitable for being attached to the barrel shaft, wherein the second portion is to be inserted into a circumferentially extending groove provided on the barrel shaft.