Neck Donut
Also: Donut
A ring of thickened brass that builds at the neck-shoulder junction over repeated firings and can pinch the bullet, sometimes turned or reamed away.
A neck donut is a band of extra brass that forms at the base of the case neck where it meets the shoulder. It develops as brass flows forward during firing and sizing, gradually thickening that junction until it stands proud on the inside of the neck. When a bullet is seated deep enough for its base to reach that ring, the donut squeezes the bullet and disturbs the consistency of neck tension.
The donut matters most when bullets are seated long, with the bearing surface extending down into the shoulder area. Shooters who chase the last bit of precision remove it by neck turning the outside or reaming the inside of the neck, and some simply keep bullets seated shallow enough to clear it. Because the donut comes from brass flow, it tends to reappear as the brass is fired and resized again over its life.