You won’t get far into firearms or ammunition before hearing the term headspace. It’s one of the most frequently used technical terms in the shooting world, and one of the least understood.

Headspace matters because it directly affects safety, reliability, and consistency. Get it wrong and the best-case outcome is poor accuracy or malfunctions. Worst case, you’re venting high-pressure gas into places it absolutely does not belong.

So let’s clear it up.

What is Headspace?

Headspace is the distance from the breech face to the surface in the chamber that stops forward movement of the cartridge.

That’s it. No mysticism required.

Every cartridge is designed to stop against a specific feature in the chamber. That feature determines how far the cartridge can move forward when chambered and, critically, where the cartridge sits relative to the firing pin, bolt face, and chamber walls at the moment of ignition.

All headspace measurements originate at the breech face, but the forward stopping point varies by cartridge design. Depending on the cartridge, headspace may be controlled by:

• The rim
• The shoulder
• A belt
• The case mouth

If you’re ever unsure which surface controls headspace for a given cartridge, stop guessing and consult the SAAMI or CIP chamber drawings. Assumptions are how people end up learning lessons the loud way.

The Four Ways Cartridges Headspace

Headspace dimensions typically can be broken down into 4 different types listed below.

1. Rimmed
2. Shoulder
3. Belt
4. Case Mouth

Each behaves a little differently, and each has its own failure modes when things go wrong.

Reading a SAAMI Headspace Callout

As a general rule, SAAMI identifies headspace dimensions with a circled “X” on the minimum chamber drawing. This marking does not appear on the maximum cartridge drawing.

CIP does not designate headspace dimensions in the same visual manner, even though cartridges governed by CIP standards headspace in the same four fundamental ways. Under CIP conventions, the controlling surfaces and dimensions must be inferred from the chamber geometry rather than identified with a specific symbol.

In the United States, SAAMI defines cartridge and chamber standards; in Europe, that role is filled by CIP.

Headspacing on the Rim

Rimmed cartridges headspace on—you guessed it—the rim.

Classic examples include:

• .45 Colt
• .30-30 Winchester
• .38 Special
• .303 British

Some modern cartridges still use rims, such as .327 Federal Magnum, .460 S&W, and .500 S&W. Rimmed designs work well in revolvers and certain rifles, but they complicate feeding in semi-automatic firearms. For that reason, they have largely fallen out of favor in self-loading designs.

On a SAAMI chamber drawing, the headspace dimension for a rimmed cartridge is measured from the breech face to the forward surface of the rim recess. This dimension is specified on the minimum chamber drawing, not on the maximum cartridge drawing.

Example: .327 Federal Magnum

The chamber print specifies that the rim recess may be as shallow as 0.056 inches and as deep as 0.070 inches. This range defines the allowable headspace in the chamber.

On the corresponding cartridge drawing, the rim thickness may range from 0.045 inches to 0.055 inches. Rim thickness is the feature that actually locates the cartridge in the chamber.

The attentive reader will notice that the maximum rim thickness of 0.055 inches and the minimum chamber recess depth of 0.056 inches result in 0.001 inch of minimum clearance between the cartridge and the chamber.

That clearance is intentional. It is required to allow the cylinder of a revolver to rotate freely without binding. This is not sloppiness or tolerance stacking, it is deliberate design.

Common Headspace Failures in Rimmed Cartridges

Rim thickness itself does not change with firing. Rims can be bent, peened, or otherwise damaged, and that can prevent a cartridge from chambering, but aside from physical damage the rim is generally a reliable and stable headspacing surface.

Case length, however, does change with firing, and that is where rimmed cartridges most often get into trouble.

This is rarely an issue with straight-wall cartridges, particularly when fired in revolvers, but it becomes far more relevant with rimmed rifle cartridges and bottleneck designs.

If a rimmed case grows beyond maximum length, the case mouth can begin to contact the transition between the end of the chamber and the start of the throat. This transitional surface is typically angled, often between 15 and 45 degrees, depending on the chamber design.

When the case mouth contacts this angled transition, it can effectively be crimped inward as the cartridge is forced into the chamber.

This unintentional crimp reduces the available volume inside the case and restricts bullet release. The result can be a significant pressure spike, and in extreme cases, damage to the firearm.

Bottom line: rimmed cartridges still require case length control.
The rim doesn’t magically save you from bad habits.

Headspacing on the Shoulder

Headspacing on the shoulder can be tricky to understand because it isn’t something that can be measured directly with a plain micrometer.

Examples of cartridges that headspace on the shoulder include:

• .223 Remington
• .308 Winchester
• .30-06 Springfield
• 5.7×28mm

It is important to understand that a bottleneck case does not automatically headspace on the shoulder.

A good example of this misconception appeared earlier with the .303 British. Although it is a bottleneck cartridge, it headspaces on the rim, not the shoulder. Another commonly misunderstood example is the .357 SIG, which headspaces on the case mouth, not the shoulder.

Design intent matters. Cartridge shape alone does not tell the full story. For that reason, it is always important to consult the cartridge and chamber drawings rather than assuming how a cartridge headspaces based on appearance.

How Shoulder Headspace Is Defined

Unlike rimmed cartridges, shoulder-headspaced cartridges are defined by two interacting dimensions:

1. Length from the breech face
2. Diameter at a specified point on the shoulder

The diameter is listed as a basic (B) dimension, identified by the circled “X” symbol. Basic dimensions are fixed reference values and are not toleranced.

Headspace is measured from the breech face to an imaginary circle, called the datum, located on the shoulder at the specified basic diameter. For .30-06 Springfield, that datum diameter is 0.375 inches.

These two dimensions work together to locate the cartridge correctly in the chamber.

Measuring Shoulder Headspace in Practice

So how do you measure shoulder headspace in the real world?

You use a comparator.

A comparator is a ring with an internal diameter that matches the shoulder datum. It attaches to a set of calipers and allows you to measure the distance from the case head to the datum point on the shoulder.

Several tools accomplish this task, but the most common and practical option for reloaders is a ring-style comparator that clips onto calipers. Multiple manufacturers offer these systems with interchangeable inserts sized for different datum diameters.

Another common method is the use of a cartridge gauge or case gauge. These gauges check overall cartridge geometry against minimum and maximum limits. If a cartridge does not fully seat in the gauge, it indicates that one or more dimensions are out of specification.

Case gauges are available for nearly every common cartridge and are a useful diagnostic tool at the bench. However, they indicate pass or fail only; they do not provide a numerical measurement of shoulder setback.

Failure Modes and Safety Concerns

If a bottleneck cartridge has insufficient effective headspace, meaning the cartridge is effectively too long for the chamber, it will usually fail to chamber. This is generally a self-limiting condition. If the bolt will not close, the rifle does not fire.

Forcing the bolt closed is not harmless.
It is a warning sign, not a workaround.

The cause of a stiff bolt during chambering is not always immediately obvious. It may be the result of:

• A partial bore or chamber obstruction
• A case that is too long, causing the case mouth to contact the throat and effectively crimp the bullet
• A bullet seated too long, contacting the rifling and being pushed back into the case
• Improper shoulder setback, requiring the bolt to force the case shoulder rearward during chambering

None of these conditions are benign, and none should be forced. A shooter can generate significant mechanical leverage when closing a bolt-action rifle, often enough to force a cartridge to chamber in all but the most stubborn cases. If something does not feel right, stop and inspect both the firearm and the ammunition.

That leverage matters. Because rifle actions provide significant mechanical advantage, forcing a cartridge into the chamber can drive the case mouth into the throat, effectively crimping the bullet in place. This reduces available case volume and delays bullet release, producing a dangerous overpressure condition that can, in severe cases, destroy the rifle.

If a firearm has excessive headspace relative to the chamber, it may still chamber and fire, but the case can stretch excessively during ignition. Over time, this stretching can lead to incipient or complete case head separation.

Excessive headspace is typically not the result of improperly sized brass. It is far more often caused by a worn, out-of-spec, or defective firearm.

When shoulder headspace is excessive and a round is fired, hot, high-pressure gas can escape rearward around the case head. This can result in blown primers, case failure, damage to the bolt or action, and, in extreme cases, damage to the firearm itself.

Headspaces off the Belt

There are a small number of cartridges that headspace on a belt of brass located just forward of the extractor groove. Functionally, belted cartridges behave very similarly to rimmed cartridges.

At present, there are no belted pistol cartridges. Belted designs are almost exclusively magnum rifle cartridges, including:

• .300 Winchester Magnum
• .375 H&H Magnum
• .416 Weatherby Magnum
• .458 Winchester Magnum
• .458 Lott

The majority of belted magnum cartridges are bottleneck designs, but they headspace from the breech face to the forward surface of the belt. The dimensional callout is treated identically to a rimmed cartridge on a SAAMI drawing.

When built strictly to SAAMI specifications, the belt is the sole feature that locates the cartridge. The remainder of the case effectively “floats” in the chamber until firing, at which point the case expands and conforms to the chamber walls.

Not everyone follows SAAMI exactly.

In the case of the .300 Winchester Magnum, some rifle builders deliberately cut the chamber so the cartridge headspaces on the shoulder, while providing clearance for the belt. This is not how the cartridge is defined in the SAAMI drawings, but when executed correctly it is safe and functional.

The original purpose of the belt was to provide a positive headspacing surface on early magnum cartridges with shallow shoulders. It was not primarily intended as a pressure-strengthening feature. Modern cartridge design has largely made the belt unnecessary, and many contemporary cartridges operate at equal or higher pressures without it.

Failure Modes in Belted Cartridges

If headspace is incorrect on a belted cartridge, the failure modes mirror those seen in other headspace systems.

A cartridge with a belt that is too thick will not chamber in a SAAMI-spec chamber.

A cartridge with a belt that is too thin, or a chamber with excessive belt clearance, can allow excessive case movement during firing. This may result in case head separation, primer failure, or damage to the firearm.

Because most belted magnums also have substantial shoulders, it is possible for a cartridge to effectively headspace on the shoulder instead of the belt. This commonly occurs during reloading when the shoulder is not set back sufficiently during resizing.

This condition may result in stiff bolt closure. If the bolt closes normally, it is unlikely to present an immediate safety hazard, and many reloaders inadvertently operate this way without incident. However, it does represent a departure from the original design intent and can affect chambering consistency and brass life.

Headspaces off the Case Mouth

Rimless, straight-wall cartridges headspace on the case mouth. Most, but not all, of these cartridges are pistol.

Because these cartridges lack a rim or belt, the case mouth is the feature that stops the cartridge from moving farther forward in the chamber. This also ensures that the bullet does not contact the rifling prior to firing, which is generally undesirable in production ammunition.

The chamber includes an abrupt step just forward of the case mouth. When chambered, the case mouth rests against this step, locating the cartridge. The case is effectively trapped between the case mouth and the breech face.

If a case is too long, it will typically fail to chamber.

If a case is too short, the cartridge may move forward when struck by the firing pin. This can result in light primer strikes, misfires, or leaking and blown primers. Escaping gas from a blown primer can damage the firearm.

Crimping Considerations

Cartridges that headspace on the case mouth must be crimped using a taper crimp, not a roll crimp.

A taper crimp gradually reduces the diameter of the case mouth just enough to remove flare and provide bullet retention, while preserving the case length required for proper headspacing.

A roll crimp, by contrast, curls the case mouth inward. On a cartridge that headspaces on the case mouth, this shortens the effective case length and can prevent proper headspacing, leading to misfires or unreliable ignition.

Why .357 SIG Does Not Headspace on the Shoulder

Although .357 SIG is a bottleneck cartridge, it does not headspace on the shoulder. It headspaces on the case mouth.

The shoulder on .357 SIG is relatively short and shallow compared to most bottleneck cartridges. If the cartridge were to headspace on the shoulder, that shoulder could flex slightly under firing pin impact, a phenomenon sometimes referred to as crush-up. This flexing can absorb firing pin energy and result in light primer strikes.

By headspacing on the case mouth instead, this potential source of ignition inconsistency is eliminated. Similar concerns can arise in other cartridges with very short or shallow shoulders, reinforcing the importance of understanding design intent rather than relying on case shape alone.

It would be incorrect to assume that every bottleneck pistol cartridge follows the same headspacing pattern, even if the cases appear similar at first glance. The .429 Desert Eagle, for example, has a broadly comparable profile but does headspace on the shoulder.

This is a good illustration of why it is important to consult cartridge and chamber drawings before loading a new cartridge.

In Closing

Headspace is not a concept everyone needs to master, but it is one that every shooter and reloader should understand at a basic level. When determining whether ammunition is safe to fire, or whether a firearm is properly chambered, headspace matters.

SAAMI and CIP use different conventions to define and display headspace dimensions. In this article, we focused on the SAAMI method. A future article will cover CIP drawings and how their dimensional callouts differ.

It is worth noting that, regardless of standard, cartridges still headspace in the same four fundamental ways. What changes is how those dimensions are presented on paper—not how the cartridge behaves in the chamber.

-Jay-

Revised 2/7/2026