HEAD SPACE – What the heck is it, and what does it mean?
Category : Reloading
In reloading, most of the time, you won’t really have to deal with “head spacing”. But, what is it? And, how does it affect me?
Here’s head spacing in a nut shell:
In order for a cartridge to properly fit in the chamber, it has to be sized to a specific dimension. Some of those dimensions can be fixed, such as the thickness of the extraction rim. In other instances, that dimension is formed and reset to a “relative position” which can often be difficult to measure easily. However, if the cartridge is made too small, or set such that one of those important dimensions is undersized, other issues can arise. So, let’s first cover the head space criteria for each of the four types of cartridges. In all instances, the head space is measured from the FACE of the closed bolt. That is, the cartridge gets installed in the chamber, and the bolt or slide (semi auto) is closed on the end of the cartridge. Let’s go over the cartridge terms again:
- Brass case – The container which becomes the assembly once the primer, powder, and bullet are installed. Of the four components, it’s the only reusable component. The case has the following parts. Case mouth – The open end where the bullet gets installed. Case Head – the end of the case where the extraction rim is.
Primer Pocket – the location in the center of the brass case where the primer is located. Center fire cartridges are called that because the primer is located in the Center of the head end. Some cases are straight walled or slightly tapered. Most rifle cases have a Shoulder. Shoulder – This tapered feature of a rifle case serves a purpose of acting as a gas seal (think piston ring in your car engine), but also is important because it is the feature that in that type of crass case is the basis for the head space. Neck – The portion of the brass case that holds onto the bullet. Flash Hole – The small hole through the case head that allows the flame from the primer to get to the inside of the case, where the powder is.
- Bullet – The projectile that travels down the barrel to the target. Some people mistakenly refer to the cartridge assembly as a bullet. The bullet is only the projectile component.
- Primer – This is the small brass incendiary component that is replaced in the case head of the brass case, and when struck by the firing pin or striker, begins the powder burning.
- Powder – the extruded, flake, or ball style combustible that is installed inside the brass case. When burned, it creates a high pressure, which sends the bullet on its way down the barrel.
Together, these four components assemble into a cartridge assembly. Again, that cartridge assembly must be able to fit inside the chamber of the gun, so that it is easily installed, and after firing, can be easily removed for replacement. BTW, brass cases are typically made from a brass alloy referred to as 70/30 Cartridge Brass. That alloy is 70% Copper and 30 % Zinc. The exact composition is a bit different than that (trace amounts of other elements are added for manufacturing purposes). The alloy has been developed because it handles the pressures of firing just fine. It expands and stretches slightly during firing, and then relaxes slightly so that it can be removed easily from the chamber. It both forms and machines well.
There has to be a bit of extra space in order for the cartridge to install easily. However, excessive space can create issues, as described earlier. So, let’s look at the four types of cartridges, and discuss that fit and head spacing detail for each:
- Rimmed Cartridge – This is the basic revolver straight walled type cartridge assembly. It’s most noticeable feature is that the extraction Rim is a larger diameter than the
diameter of the sides of the case. These cartridges head space on the thickness of the rim. As long as the overall length of the cartridge, including the installed projectile, will fit and clear the rotation of the cylinder, the head spacing for this type of cartridge is fixed in the manufacturing of the brass case.
- Rimless Cartridge – This is the semi auto style pistol cartridge case. It’s noticeable feature is that the rim diameter is the same as the body diameter of the brass case. This type of case would just fall through a revolver cylinder since it has no feature to keep it from doing so. Some revolvers chambered for this type of cartridge use what is called a “moon clip” to snap the cartridges onto. It’s the only legitimate use of the term “clip” in hand guns. The stacking part of the gun that holds multiple rimless cartridges is correctly called a “magazine”. Since these cases need a locating feature, that feature is the end of the case mouth. If you look inside the chamber of a semi
auto pistol, there is a small “lip” located at the front of the chamber. The case mouth of the brass case locates against that lip. That is the head spacing for a rimless cartridge.
- Shouldered or Bottle Necked Rifle Cartridge – Like the rimless semi auto cases, the rims of these cartridges are the same diameter as the body diameter. However, the neck diameter gets reduced to create a pneumatic high pressure feature that sends the bullet faster than it would otherwise. The reduction from full diameter to neck diameter creates a shoulder that can act to seal gasses that would otherwise flow backwards into the chamber. Those burning gasses are of a very high (virtually plasma) temperature, and would erode the metal of the chamber if allowed to flow backwards freely. So, that tapered shoulder acts in two regards. First, and most importantly, as a gas seal. Secondarily, it acts as the locator or head space in the chamber. The head space on shouldered rimless rifle cartridges is from the closed bolt face to the beginning of the taper on the shoulder. 4) Belted “Magnum” style shouldered rifle cartridges – Because of the extreme pressures of this style of cartridge assembly, the manufacturers build in a “belt” to provide additional pressure support at the back of the brass case. In this style of brass case, the thickness of the belt serves as the head spacing detail. However, this style of cartridge creates a special situation for reloaders since the shoulder must also act as a gas seal, as in the instance of the aforementioned shouldered rimless rifle cartridge.
So, we have discussed the basics of head space, and how it’s measured or used in your chamber. But, what are the possible problems if the head spacing of your chamber is wrong? For reloaders, there are two parts to this equation.
First, the chamber must be properly made, and adjusted in the gun to accept a properly made cartridge assembly. Older guns, especially older service rifles, can have had a tough life of hard use, and little cleaning. Those older and worn out rifles not only suffer from poor barrel quality (lands and grooves worn). They can also have excessive head space in the chamber. A cartridge, especially one loaded to very high pressures, can get easily installed in the chamber. But, when fired, the brass case SLAMS backwards against the bolt, often violently, and can cause incremental damage to the rifle. Excessive head space will certainly at least severely reduce the life of the reloadable brass.
New guns are typically OK with regard to head space settings in the chamber. Custom guns should be made by competent gun smiths to assure that the head spacing is set correctly.
Secondly, the reloading must be correct to accommodate the properly head spaced chamber. I would refer you to the articles on the subject of reloading for the shouldered and belted types of rifle cartridges. Those cartridge types require special consideration for setting reloading tools. The rimmed and rimless pistol cartridge tools have reloading considerations built into the reloading tools. Trimming for those two types is often not needed. However, the shouldered rifle cartridges, after full length sizing, and adjusting head spacing and shoulder location, will almost always require some trimming to get the assembled cartridge correct. Reloading brass (with regard to head spacing) to an UNDERSIZED condition is just as bad as having a chamber that is oversized (with regard to head spacing).