The term "headspacing" is defined as the distance between the face of the bolt and a point in the chamber that prevents further forward movement of a cartridge.
Click on image to enlarge. Public domain image.
In the above pictures, we have a .45 ACP cartridge loaded into a chamber. The cartridge case is a bit larger than the bullet and the chamber is shaped so that it fits the case correctly. Of course, depending on the shape of the cartridge, the headspacing could be different for different firearms.
For instance, some of the earliest metallic cartridges were rimmed. This means, they have a rim at the base of the cartridge that is larger than the diameter of the cartridge case. When this type of cartridge is pushed into a firearm's chamber, the rim positions the cartridge correctly and prevents it from slipping too far into the chamber. Therefore, for a rimmed cartridge, the proper headspacing should be the width of the rim.
Headspacing of a rimmed cartridge. Click on image to enlarge.
The most popular cartridge today, the .22 Long Rifle rimfire cartridge is an example of a rimmed cartridge.
However, it must be noted that most modern cartridges are rimless (i.e.) the diameter of the rim is the same or smaller than the diameter of the case body. Modern cartridges have other ways to ensure that the bullet seats in the chamber correctly. For example, for most pistol cartridges, the case diameter is slightly larger than the bullet and the chamber has a shoulder that the case mouth rests against.
Headspacing on a .45 ACP rimless cartridge. Click on image to enlarge.
In the image above, we see a .45 ACP rimless cartridge seated in a chamber. The case mouth is larger than the bullet diameter and rests against the front of the chamber, thereby correctly seating the cartridge.
Other rimless cartridges, such as bottle-necked cartridges (which are mostly used by rifles), the shape of the firearm's chamber is correspondingly tapered to achieve proper headspacing.
Headspacing on a bottlenecked rimless cartridge. Click on image to enlarge.
In the above image, we see that the chamber is tapered to accommodate the cartridge and the headspacing is the distance from the bolt face to the tapered section.
In the case of belted cartridges (such as the hunting cartridge, Holland & Holland .300 Magnum), the chamber is shaped to seat the forward face of the belt.
Headspacing on a belted cartridge. Click on image to enlarge.
When the H&H .300 Magnum was first being developed, it was difficult to provide proper headspacing for a cartridge with a shallow shoulder design. The solution was to add a belt around the cartridge body. This is similar in function to the rim of a rimmed cartridge, but gave a long enough surface for the cartridges to fit side-by-side in a magazine, without risk of interference during the feed stroke. This idea was later copied by other magnum rifle cartridges as well.
We will study the importance of proper headspacing in the next post.