Cartridge Types
A range of small arms cartridges have a flange at the base of the case called a rim. A part of the firearm called an extractor hooks onto the rim to pull the spent case out of the chamber after it has been fired. The cartridge is said to be rimmed when the rim is of larger diameter than the remainder of the case. Most early cartridges were rimmed, and the rim-stop ledge at the rear of the chamber prevents the cartridge from moving forward. Headspace for rimmed cartridges is the thickness of that rim..
The cartridge is said to be rimless if the extractor groove is machined into the body of the case so the rim is the same diameter as the adjacent part of the case. Most modern automatic weapons use rimless cartridges. Straight rimless cartridges may headspace on the forward edge of the cartridge as shown in the diagram at the top of this article. Bottle-necked rimless cartridges may headspace on the conical shoulder between the narrow neck and the larger diameter portion of the case.
Some large rimless magnum or military cartridges have a belt machined above the extractor groove. This belt is of slightly larger diameter than the adjacent case, so the cartridge may headspace on the edge of the belt closest to the bullet.
Read more about this topic: Headspace (firearms)
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