Headspace Disclaimer

Put simply, headspace is not as simple as it appears on the surface.

A Brief Introduction to Headspace

What is headspace?

If you’ve read our Sister Cartridges Deep Dive, you should know what headspace is and why it is so important. To recap:

Headspace is a measurement of chamber length from the bolt face to a mid point on the shoulder called the datum line.

Headspace basically describes how much room the case has inside the chamber. There is both a lower and an upper limit to headspace, for the following reasons:

A short headspace may result in the bolt crushing the case, leading to dangerous over-pressurization when a cartridge is fired.
A long headspace may result in excessive stretching and rupture of the case when a cartridge is fired.

How is headspace measured?

We measure headspace using a headspace gauge. A headspace gauge is a precisely-manufactured representation of a case of a particular length, which translates to the corresponding chamber length.

A minimum (GO) headspace gauge represents the shortest safe chamber length for the cartridge.
A maximum (NO GO, FIELD) headspace gauge represents the longest safe chamber dimension for the cartridge.

To measure headspace, the applicable gauge is selected and inserted into the chamber. A bolt head (with no extractor or ejector installed) is then inserted into the barrel extension and twisted.

GO/Minimum: If the bolt does not rotate/lock, the chamber headspace is too short.
NO GO/FIELD/Maximum: If the bolt does rotate/lock, the chamber headspace is too long.

This is where conventional understanding of headspace typically ends.

Beyond the Barrel: Bolt Dimensions

We will only touch on the bolt dimensions briefly, to acknowledge that in some cases, headspace issues can also result from an out of specification bolt head. There are 2 key bolt head dimensions that can affect headspace measurement:

Bolt Lug Height: If the lugs are too tall, the entire bolt will need to sit deeper into the barrel extension to close (decreasing the effective chamber length). Conversely, short bolt lugs will have the opposite effect (increasing the effective chamber length).
Bolt Face Depth: If the bolt face is too shallow, the bolt will try to push the cartridge (or headspace gauge) deeper into the chamber (decreasing the effective chamber length). Conversely, a deep bolt face will have the opposite effect (increasing the effective chamber length).

We will not get into the Colt Technical Data Package (TDP) specifications for the bolt head. Suffice to say, you should only buy high quality BCGs from a quality manufacturer.

HINT: If you don’t have the instruments and specifications to gauge your bolt heads, check headspace with multiple bolt heads from different high quality manufacturers. Odds are, at least one of those bolts is in spec. If a barrel fails headspace with two bolts, its probably the chamber, rather than the bolt.

The fact that not every bolt is cut to the same dimensions suggests the importance of checking headspace with the bolt that will be paired with the barrel. Just because a barrel passes headspace with my bolt doesn’t mean it will pass with yours.

Gauge Bias

Metrology is the science of measurement and is a core component of quality control. Metrology and quality control utilize instruments and standards to evaluate a subject or product against a known comparator.

A headspace gauge is a type of dimensional standard. These gauges are carefully manufactured to exact dimensions within a specific tolerance (the allowable range, or error, around the target value).

Tolerance may be distributed evenly around a target (i.e. a normal bell curve distribution with neutral bias), or may be biased in one direction (e.g. Poisson distribution).

Bias describes asymmetrical error relative to the target value. Positive bias means that the error is focused above the target. Negative bias means the error is focused below the target. Directionality of gauge bias plays an important part in quality control.

If we consider the application of gauge bias to headspace gauges:

A minimum headspace gauge should have a positive bias. The Forster Colt-spec 5.56 NATO Minimum headspace gauge is manufactured to 1.4646″ -0.0000″ / +0.0003″.
A maximum headspace gauge should have a negative bias. The Forster 5.56 NATO Maximum headspace gauge is manufactured to 1.4736″ -0.0003″ / +0.0000″.

This principle and application of gauge bias for dimensional standards is conserved in every industry where dimensions matter. By following these principles of quality control, you will never pass defective product (i.e. undersized or oversized chambers). However, because the gauge bias errs on the side of caution, you may reject conforming product (i.e. chambers close to the lower or upper limits). Burn this concept into your brain.

Why Does This Matter?

Some barrel manufacturers ream the chamber as close as possible to the minimum headspace. This is a good thing. Over time, the chamber will lengthen due to erosion, corrosion, stretching, etc. The closer to the minimum, the longer the barrel should last before it fails the maximum headspace specification.

Ideally, the length of a headspace gauge would be exactly as long as it says it is, with zero error. However, there is always error in test standards (a higher grade gauge will just have smaller error). Good quality control practices dictate that error bias should favor passing only good product at the expense of also rejecting some good product near the specification limits. This means that minimum gauges may be larger than the nominal value (MUST NOT be smaller), and maximum gauges may be smaller than the nominal value (MUST NOT be larger).

Get to the point…

A perfectly reamed chamber (reamed to the minimum depth) will likely fail minimum headspace verification. This is because the gauge used to verify the headspace is almost certainly above the nominal value, for the reasons indicated above. So, while manufacturing the chamber close to the lower limit contributes to a longer life, doing so also increases the risk of false rejection (rejecting conforming product).

As an example, we will look at headspace verification of a .223 Wylde Criterion barrel using a Forster headspace gauge.

The .223 Wylde chamber has the headspace of a 5.56 NATO chamber (1.4646″), with a throat geometry closer to the .223 Remington chamber. A .223 Wylde chamber should pass the Colt-spec 5.56 NATO headspace verification.
Criterion manufactures to extremely tight tolerances. It is not unusual for a barrel coming off the Criterion line to be reamed to a headspace of 1.4646″.
Forster GO/minimum headspace gauges, which are an industry standard, have a positive bias. The Colt-spec 5.56 NATO gauge has a tolerance of 1.4646″ -0.0000″ / +0.0003″. That means the gauge may have a headspace equivalent length of anywhere between 1.4646″ and 1.4649″.
In theory, when you put a labeled 1.4646″ headspace gauge in a 5.56 NATO/.223 Wylde chamber and install a bolt that meets the specifications of the TDP, the bolt should turn in the barrel extension. However, gauge bias complicates things a bit.

The matrix below illustrates the possible scenarios for a chamber reamed short (1.4640″), perfectly (1.4646″), and long (1.4650″) evaluated using a headspace gauge that is on target (1.4646″) and on the positive bias (1.4649″).

If the barrel and gauge are both perfect, the barrel will correctly pass the minimum headspace verification.
If the chamber is reamed short, the barrel will correctly fail the minimum headspace verification.
If the chamber is reamed long, the barrel will correctly pass the minimum headspace verification.
If the barrel is perfect and the headspace gauge is on the positive bias (which is very likely), the barrel will appear to fail the minimum headspace verification. A 1.4649″ long headspace gauge in a 1.4646″ chamber will not allow the bolt to rotate in the barrel extension.

What Does It Mean?

Don’t drive yourself crazy measuring headspace.

Don’t get us wrong; headspace is very important. A truly undersized chamber is dangerous. A truly oversized chamber is dangerous. But if you don’t understand the principles of metrology, quality control, and manufacturing, you may find yourself unhappy with the most perfect product, in arguments with manufacturers (who probably know better than you), and completely wrong.

When you headspace enough barrels, you will know what “barely failing” feels like. If the bolt is almost able to close on a 1.4646″ gauge, and spins freely on the 1.4636″ gauge, either the chamber is a couple ten thousandths too short for the gauge or the gauge is a couple ten thousandths too long for the chamber. Given the principle of gauge bias, the most likely explanation for “barely failing” minimum headspace is a positive bias on the gauge and a nearly perfect chamber.

If you toss a “barely failing” barrel due to minimum headspace, you are most likely tossing a perfectly good barrel.

It is worth noting that a barrel “barely passing” the maximum headspace is, at best, near the end of its useful life (regardless of whether it is new or not).

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