Is the shellholder specific to caliber or to the dimensions of the case head?

The correct interpretation is that the shellholder is specific to the dimensions of the case head. This specificity arises because the shellholder is designed to securely grasp the base of the cartridge case during reloading processes.

The physical characteristics of the case head, such as its overall diameter and the shape of the extraction groove, determine how well the shellholder will fit and hold the cartridge. Each shellholder model is engineered to accommodate particular head dimensions that correspond to a range of cartridges.

While caliber can influence the type of cartridge used, it does not directly determine the fitting of the shellholder. For example, different calibers may share similar case head dimensions or have various case designs that could require different shellholders to fit correctly. The emphasis on the case head dimensions ensures that the shellholder properly interfaces with the cartridge to provide a stable foundation during resizing or seating operations.