Mandrel
Also: Expander Mandrel
A precision rod that sets case-neck inside diameter from within, an alternative to a sizing bushing for establishing consistent neck tension.
A mandrel is a smooth, precisely ground rod that you push into a sized case neck to set its inside diameter from the inside out. Used this way, it offers a different route to consistent neck tension than working purely with a bushing die, which sizes the neck from the outside. Many handloaders size the neck slightly small, then run a mandrel through it so the final inside dimension, the one that actually grips the bullet, is set by the mandrel.
The argument for working from the inside is that it tends to push any neck-thickness variation outward rather than inward, which can leave the inside diameter more uniform from case to case. A mandrel is also commonly used as an expander step ahead of neck turning to give a true reference surface. It is a small tool, but it changes how the rest of your reloading dies interact with the brass.