Headspace Control

Spudy says... "Controlling headspace is critical for a safe working gun. In firearms, headspace is measured from a closed chamber's breech face to the chamber feature that limits the cartridge's insertion depth. We can control this when we full-length size our brass by measuring it before and after sizing with a full-length die. You can not alter the headspace with a neck sizing-only die. Correct headspace will also aid a round to chamber and  the bolt to drop more smoothly and correctly than a case that's been only neck sized repeatedly."