Neck sizing dies size only the case neck of the cartridge - not the shoulder or the case body. Use neck dies when cases are fired in the same rifle and a close cartridge case to chamber fit is desired. The neck die will size the neck, decap the spent primer, then size for correct bullet tension with the expander ball. Neck dies come in an RCBS plastic die box and will fit all 7/8 to 14 reloading presses.
Sinclair, Issue:5A, Page:055