Timepiece movement having a barrel with reduced core diameter

A timepiece movement comprising:

• at least one plate at a fixed distance of centers from a bridge jointly carrying a timepiece barrel assembly including at least one barrel spring mounted between, at a first end, a barrel drum arranged beside the bridge, and at a second end, a receiving surface comprised in a barrel core rotating integrally with a ratchet and coaxial to the drum about a common pivot axis, the barrel assembly including on the plate side a cover fixed to the drum, the cover and the drum together forming a chamber confining the spring,
• wherein the spring is made of a multiphase, cobalt-nickel-chromium based alloy, comprising 44 to 46% cobalt, 20 to 22% nickel, 17 to 19% chromium, 4 to 6% iron, 3 to 5% tungsten, 3 to 5% molybdenum, 0 to 2% titanium, 0 to 1% beryllium, and having a Young's modulus of between 200 and 240 GPa and a shear modulus of between 80 and 100 GPa, and having a width to thickness ratio of between 9 and 21,
• wherein maximum thickness of the steel or stainless steel core relative to the pivot axis is less than nine times maximum thickness of the spring, and
• wherein the movement further comprises a shake limiting means including at least one shim-washer, which is independent of the core and guided by a shoulder of the core and inserted between the plate or the bridge respectively and the cover or the drum respectively.