What Is Headspace?

So first off, what does headspace actually mean? Headspace is the distance from the face of the bolt (breech face) to the point in the chamber designed to contact the cartridge case. In most bottleneck cartridges, this is the midpoint of the shoulder angle. See below.

Theoretically, this is the only point before firing that the case actually contacts the chamber, because the cartridge is slightly smaller than the chamber so that it can be inserted without interference. So headspace essentially comes down to how much room you have between the bolt face and the contact point in the chamber. That’s why it’s such a critical dimension when it comes to firearm manufacturing.

Why Headspace Matters

Because headspace controls how far the cartridge can sit into the chamber, it controls how the case sits in relation to the bolt. Because of imperfect manufacturing methods, everything has some variance, so each cartridge has its own acceptable limit that the headspace must fall between. This also means that no cartridge is going to sit perfectly on the bolt and in the chamber. This is why a “tight” headspace is usually considered more accurate — it allows less variation for the cartridge sitting in the chamber. Some firearm manufacturers hold to tighter tolerances than the typical standard for this reason.

What Happens When Headspace Is Out of Tolerance

If the headspace is too deep, the cartridge can go further into the chamber, which allows the cartridge to sit farther off the bolt face. This means the firing pin won’t be able to indent as far into the primer, and the case has to stretch more after firing to elongate and fill the chamber. This could possibly result in light primer strikes and over-stretching the brass, potentially leading to case failure in a worst-case scenario.

If the headspace is too tight, the case can’t be pushed into the chamber far enough, meaning the bolt can’t move all the way forward into position. This can result in very difficult bolt close, because the case shoulder is being compressed, or the inability to close the bolt at all.

How Headspace Issues Are Caught

Because this is such a critical part of the function of the firearm, it’s checked multiple times by the manufacturer. Actual headspace issues are extremely rare, but they can be diagnosed and fixed by a competent gunsmith if found. There are also two parts to this: the rifle’s headspace must be within established tolerances, but the case must also be within appropriate tolerances. Improperly sized cases will show the same signs as headspace issues — this is especially important if you hand load ammunition.

Common Headspace Methods by Case Type

Above we mentioned the bottleneck cartridge, but there are several other case variations that have different methods of setting headspace. Although the methods and locating features are different, the purpose is still the same. Here’s an overview of some of the common methods.

Bottleneck

This is the style shown above. Most modern rifle cartridges have adopted this case design. The headspace is set off the midpoint of the shoulder taper. Because this is on the neck, the case is centered into a cone in the chamber. This cone helps align the case to the bore, ensuring everything is lined up consistently while also being used to control the headspace. This is generally considered the most accurate for that reason — because of the geometry, the case self-centers in the chamber, leading to more consistent results. That doesn’t mean there aren’t very accurate rifles with the other styles, though.

Rimmed

This is very common in revolver, lever action, and rimfire cartridges. A rim with a larger diameter than the case body sits at the base of the case. For these, the headspace is defined as the distance from the breech face to essentially the face of the barrel. Because the rim is the contacting factor, the headspace is defined around the thickness of the cartridge rim.

Belted

This style is common in some large magnum cartridges, such as the .300 Win Mag and the Weatherby cartridges. The headspace on these is controlled by the belt at the base of the case. Similar to the rimmed style, there’s a portion of the case with a larger diameter that’s the contact surface, and the base of the belt is where the headspace is set from. Even though most of these cartridges do have a bottleneck shape, that’s not where the case contacts the chamber. Since the case is bottoming out on the belt, there’s potential for the case to sit slightly off-center within the chamber tolerances — the belt keeps the case from going in further, but does nothing to help center it. Not many new cartridges are coming out with this style, but due to the popularity of some cartridges that have it, they’ll be around for a while.

Straight Wall

This style is typical of handgun rounds, but some rifle rounds use it as well. With no bottleneck, rim, or belt, the only location to contact the chamber is the case mouth — the small lip at the end of the case where it steps down to the bullet. This style also only controls the depth and does nothing to help center the case in the chamber.

Personally, I’m partial to the bottleneck style, just because it assists in centering the case in the chamber. But I’ve shot many belted and rimmed case rifles that shot amazing. With straight wall being usually reserved for handgun rounds, the accuracy differences for that style are something I haven’t focused on. Overall, headspace is something that the typical rifle owner will most likely never have to deal with, but it’s a critical part of the overall function of the firearm.