The die set is an integral part of the reloading press which performs all of the functions for reloading a cartridge. A die set may consist of two, three, or four separate dies. Some dies perform multiple actions which allow for fewer dies on the press. There are also optional dies available for certain reloading press types and brands. Optional dies are not necessary for the reloading process but allow for quicker reloads. A reloading die set is cartridge caliber specific and should only be used with the caliber information etched on the die. Typical reloading dies are threaded because they need to be adjusted for proper cartridge assembly.

This is a typical setup for a reloading pistol ammunition on a progressive reloading press.

Dies are inserted into the frame of the progressive reloading press in a circular pattern. The dies should be installed in sequential order in the direction indicated in the user's manual. This image depicts the die configuration on a Hornady progressive reloading press set up for pistol cartridge reloading. A cartridge case is fed onto the shell plate starting the reloading cycle. The cartridge case is then sized and deprimed, fitted with a primer, expanded and filled with powder, fed a bullet, seated and crimped to the bullet, and ejected from the machine.

This is the first die used in the set. The empty cartridge case is thrust into the die allowing the walls of the die to shrink it back to its factory dimension. Simultaneously, the steel pin in the center of the die ejects the used primer from the cartridge case.

The second die in the set expands the rim of the straight walled pistol cartridge case as it is thrust into the die in order to accommodate a bullet. Some expanding dies also allow the user to insert gun powder into the cartridge case at this time either by hand or automatically with a powder hopper. Powder hoppers are common with progressive reloading presses.

The powder cop die is an optional die made by Hornady. There are dies made by other manufacturers which perform the same function. The purpose of this die is to alert the user as to whether or not the right amount of powder has been placed into the cartridge case following the powder/expanding stage. Like the other dies in the set, this die must be calibrated by the user prior to reloading cartridges. On this particular die a rod is located at its center with a rubber o-ring attached. The cartridge case is again inserted into the die moving the rod vertically. The idea is to have the rubber o-ring flush with the top of the die. Should the o-ring extend above the top of the die, too much powder exists in the cartridge case. If the o-ring is not visible after inserting the cartridge case into the die, there isn’t enough powder in the case.

The bullet feeder die simply aligns the bullet with the cartridge case and places it within the newly expanded cartridge case. On a progressive reloading press, the bullets are fed through a tube into the bullet feeder die like the picture on the right. If this attachment does not exist, the user will manually place a bullet into the die by dropping it into the open top.

The seat/crimp action can be performed using one or two dies. More often with a progressive reloading press there will be one die to both seat and crimp the bullet into the cartridge case. Once the bullet has been set atop the cartridge case with the bullet feeder die, the cartridge case then enters the seat/crimp die where the bullet is seated into the cartridge case at a depth established by the user. This same die will also crimp the bullet into place in order to prevent it from coming back out or being pushed further into the case.

The crimp is what holds the bullet in the cartridge case. There are two types of crimps which serve two different purposes. A taper crimp uses friction to keep the bullet in place, while a roll crimp is more mechanical as it pushes the edge of the cartridge case into the bullet groove (cannelure) effectively locking it into place. While the roll crimp is more effective in keeping the bullet in place, a taper crimp is necessary for cartridges being fired from semiautomatic pistols. Whether the cartridge is headspaced on the rim or the case mouth or the case rim will determine which crimp to use. Semiautomatic pistol cartridges headspace on the case mouth and so the case mouth cannot be deformed as it is with a roll crimp.

Note
Do not attempt to roll crimp a cartridge case with a smooth bullet. The bullet should have a groove (cannelure) in which to apply the roll crimp.

When all of the dies are installed and adjusted within the frame of the progressive reloading press it will look something like this image.