Necked-Down
Also: Necked-Up
Reforming a parent case to accept a smaller (or larger) bullet diameter, one of the most common paths to a wildcat cartridge.
Necking down means squeezing the case neck of an existing case to a smaller inside diameter so it holds a thinner bullet, while necking up opens the mouth to take a fatter one. The body and head of the parent case stay essentially the same, so the new round keeps its case capacity and bolt-face dimensions while changing only the projectile it launches. The work is done by running the case through a sizing die built for the new diameter, sometimes in more than one step.
This is the backbone of wildcat cartridge design, since a single popular parent can spawn a whole family of rounds across different calibers. The 6.5 Creedmoor and .22-250 Remington both began as necked cases, and the Ackley Improved approach often combines necking with a sharper shoulder. Necking changes neck thickness and sometimes leaves a small step or donut at the shoulder junction, so reformed brass usually needs trimming and inspection before it is loaded.