Timepiece barrel with reduced core diameter

A timepiece barrel assembly, including at least one barrel mainspring mounted between, at a first end a barrel drum, and at a second end, a receiving surface comprised in a barrel core coaxial to said drum about a pivot axis, wherein said 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, and wherein said core is made of steel or stainless steel, and wherein the maximum radius of said core relative to said pivot axis is less than ten times the maximum thickness of said spring, and further wherein said core includes in series, from a first end towards a second end:

• a means of fixing a ratchet, and/or a first pivot shoulder in a plate or a bridge of a timepiece mechanism,
• then a pivot shoulder relative to a bore of said drum,
• then a collar radially projecting relative to said shoulder and cooperating in abutment with an inner face of said drum on the side of said at least one spring,
• then, flush with or set back radially relative to said collar, said receiving surface for said at least one spring,
• then, set back radially relative to said receiving surface, a pivot shoulder relative to a bore in a cover arranged to form, with said drum, a chamber containing said at least one mainspring,
• then a second pivot shoulder in a plate or a bridge of said timepiece mechanism.