The headspace in a resizing die is the most critical dimension and needs to be checked, and then verified by another person as a confirming check.
Headspace in reloading equipment is the distance from the base of the shell holder to a point inside the resizing die. This distance is the same as specified in the SAAMI specifications for a cartridge of the same caliber.
If the headspace is long, or too deep into the die, when the case is resized the cartridge will be long and the action will not close on the cartridge. This results in an unhappy customer, and probably a returned die.
If the headspace is short in a die, the cartridge ends up being short. It will enter the rifle action. This is where the real trouble begins. The firing pin snaps forward hitting the primer, but because the case is shorter than the rifle chamber, the cartridge is pushed forward to the shoulder in the barrel. The primer is exploding, igniting the powder. The case expands tight to the chamber wall, but the head is not tight against the breech face, so the pressure within the cartridge stretches the case until the base of the cartridge is now back against the breech face. In some instances this stretching is enough to cause the case to rupture. The expanding gasses are then released into the action, magazine, and stock interior causing any one or all to explode.
Check the head space, get it verified.
