Case Neck
The forward portion of a bottleneck cartridge case that grips the bullet and where tension, turning, and annealing are managed for accuracy.
The case neck is the thin cylindrical section at the mouth of a bottleneck case that holds the bullet in place. The grip it exerts, known as neck tension, determines how much resistance the bullet meets on release, and consistent tension from round to round is one of the strongest drivers of low velocity spread. Neck wall thickness and concentricity also affect how squarely the bullet sits relative to the bore.
Because the neck is worked hardest during firing and resizing, precision handloaders give it the most attention. Neck turning shaves the outside wall to a uniform thickness for even tension and chamber fit, while annealing softens the brass periodically to keep that tension stable and extend case life. Neglected necks grow hard and brittle, which makes tension drift and eventually splits the brass.