Redding 32 Winchester Spec. Neck Sizing Dies Neck Sizing Dies are a special resizing die that leaves the cartridge body fire-formed and only resizes the case Neck (Not recommended for reloading ammunition to be used in single shot firearms). Advanced reloaders will use a Neck sizing die to produce ammunition that is custom "fire-form" fit to their chamber. We call this type of resizing "advanced" because if you don't know what you're doing, you can reload ammunition that fits and functions poorly in your gun. .