During the neck sizing operation, what presses the sides of the cartridge against the walls of the die?

In the context of neck sizing operations during reloading, obturating is the correct term because it refers to the process where a bullet partially or completely fills the bore of the firearm during firing. However, in terms of the mechanics of placing a cartridge into a sizing die, the action that effectively presses the sides of the cartridge against the walls of the die is due to the sizing process itself.

During neck sizing, the cartridge case is inserted into a die that has an internal diameter smaller than the case neck. As the case is pushed into the die, the case neck is resized, which occurs because the die applies pressure uniformly around the neck of the cartridge. This results in the brass being formed to a tighter tolerance, ensuring that the neck diameter matches the dimensions of the bullet being used.

In this setting, while obturating may describe the effects observed when a round is fired, sizing specifically describes the mechanical action taking place during the neck sizing operation, which is essential for achieving proper bullet seating and consistent ballistic performance.