Timepiece barrel assembly, including a mainspring mounted between a drum and a receiving surface of a core coaxial to the drum about an axis. The 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; a pivot shoulder of the drum; a collar radially projecting relative to the shoulder and resting on an inner face of the drum towards the spring; set back radially relative to the collar, the receiving surface; set back radially relative to the receiving surface, a pivot shoulder of a cover for closing the barrel; and a second pivot shoulder in a plate.