Headspace refers to the space in a cartridge or housing that surrounds the head of the bullet. This space may be defined as the exterior walls or simply the empty space left between the primer and bullet. While headspace in a firearm is considered a negative space, this term can be used with greater latitude depending on the situation. In general, headspace is viewed as negative because it represents space lost from the bullet’s path through the gun’s bore or case.
A firearm has many moving parts that require space to keep them aligned and functioning properly. Each part needs an appropriate amount of headspace to ensure proper operation, including a bolt carrier, the bolt, the pistol trigger, and the magazine spring. Magazine chambers also typically have restricted headspace because the chamber must be able to accept the magazine and maintain its capacity. As previously stated, this all adds up to reduced headroom.
One way manufacturers determine headspace is by using a no-go gauge. A no-go gauge is simply a tapered string that measures the inside of the chamber as well as the velocity of a bullet from the primer to the bullet as it leaves the chamber after firing. The no-go gauge will show any areas that have increased headspace. This makes it easier for manufacturers to determine if they have increased headspace and, consequently, reduce drag during firing.
Another way to define headspace is by using a guffey. A guffey is a plastic tube that attaches to the end of the primer. When a bullet leaves the chamber, the tube expands slightly as the bullet moves down the barrel. A zero clearance test, which uses a guffey to determine headspace, determines if the round meets or exceeds the accepted limits by the bore diameter and bullet velocity at the time of impact.
A bullet that leaves the chamber too soon after firing will create an increase in headspace. In order to meet G CAP or headspace requirements, manufacturers must include a guffey in the assembly. However, there are some cartridges that do not need a guffey, such as pistol cartridges, whose interior diameter and bullet velocity will not change when fired from a pistol cartridge. Other manufacturers will incorporate the use of a guffey into their design, but will not include a separate one for the pistol cartridge since the entire assembly will fit into the pistol’s magazine well.
Some manufacturers may use a two-piece bullet that has two layers. These cartridges will have an outer shell that is made of either lead or copper and an inner liner made of either lead or brass. To determine whether a cartridge meets the criteria for headspace, it is necessary to take a look at the interior surface of the bullet and see how it compares with the bore diameter. The two measurements, or interior surface and bore diameter, can be compared on graph paper to get a horizontal line that represents the difference between the inner surface and the exterior surface of the cartridge. This line, called a pinpoint zero, represents the inside diameter of the cartridge, while the top of the zero will represent the inside of the headspace. When calculating G CAP or headspace measurements, the pinpoint zero should always be higher than the top of the headspace gauge.
There are many other measurements and specifications that go into G CAP and headspace measurements. One must keep in mind that when pressures are measured in pounds per square inch or pounds per shot, they are generally in reference to shots from a standard distance using a standardized shooting formula. The pressures at which these measurements are recorded are extremely small, so they are easy to determine. They are also often used to measure different stages of bullet hardness or softness.