Neck Tension
The interference grip the sized case neck holds on the bullet, a key influence on how consistently each round releases and how uniform velocity stays.
Neck tension is the amount of squeeze the case neck applies to the seated bullet, created when the sizing die reduces the neck below bullet diameter so the brass springs back tight. That grip controls how much pressure must build before the bullet breaks free and starts moving, which directly shapes the early part of the burn. Even, repeatable tension is one reason careful handloaders see low extreme spread across a string.
Tension depends on brass hardness, neck wall thickness, and how far the die sizes the neck down, so all three must stay consistent for results to repeat. Many precision reloaders set tension deliberately with a bushing die, choosing a bushing that leaves a known amount of interference. A separate crimp can add holding force for hard-recoiling or repeater-fed loads, though many bench shooters rely on neck tension alone.