Gas System Length
This is another one of those concepts that have inaccurate diagrams and conceptions floating around. One would think that if you are going to spend the time to create a diagram, you would actually confirm that what you are diagramming is correct. Yet, one of the most frequently used diagrams to illustrate gas system length is embarrassingly wrong.
We also want to align on true gas system length, as this is not the same as the values typically associated with nominal gas system length.
What is Gas System Length?
The gas system length describes to the distance from the chamber to the gas port.
Gas system length is most commonly described numerically as the distance between the front of the barrel extension flange to the center of the gas port. For those who like to reference the completely wrong diagram: break out a tape measure and feel embarrassed.
There are four gas system lengths that you will encounter most frequently. The diagram below illustrates the common values associated with the nominal lengths.
The nominal gas system length also defines the length of the gas tube, which reaches from the gas block into the upper receiver and into the gas key.
There are some other gas system lengths, including Micro Pistol, Intermediate, Rifle+2 (inches) and some oddball lengths. But these four are the ones you are most likely to encounter.
Why is Gas System Length Important?
While gas system length is a barrel specification, it has tremendous impact on the function of the operating system (principally, the gas system).
The two primary effects of gas system length are the pressure that enters the gas system and the time between ignition of the primer and the flow of gas into the gas system. For more information about the effect of gas system length on the function of the gas system, check out our Gas System Deep Dive.
Gas system length combines with barrel length to impact dwell time, which we will cover in a subsequent Barrel Deep Dive article.
How to Determine Gas System Length
You can determine your gas system length very easily. With the barrel uninstalled and the gas block removed, measure the distance between the front of the barrel extension flange to the gas port. Take this measurement and reference the table above to find the nominal length. Note that there is enough of a difference between nominal gas system lengths that you don’t need to remove the barrel and gas block to estimate the distance.
However, this arbitrary relationship between two visible features on the outside of the barrel is really only useful for knowing what nominal length system you have. This measurement is not particularly useful for anything else, by itself.
In order to relate the gas system length to the barrel length, we need to have a common point of reference. As discussed in our Barrel Length article, the barrel length is measured from face of the bolt. So, this is our common reference point. However, there is no external feature that corresponds with this location.
If you recall from the Barrel Length article, we measure barrel length using a dowel fed down the bore of the barrel. To measure the gas system length in the same terms, we need to use similar methodology and a little arithmetic.
- With the bolt locked in the barrel extension, run a dowl down the bore until it touches the bolt face (be sure it is not resting on the ejector, instead of the bolt face).
- Mark the dowel where it emerges from the muzzle.
- Remove the dowel and measure the length of the dowel that was in the bore. This is your barrel length.
- Insert a pin gauge through the gas port and into the bore.
- Run the dowel down the bore until it touches the pin gauge.
- Mark the dowel where it emerges from the muzzle.
- Remove the dowel and measure the length of the dowel that was in the bore. This is your dwell length.
- The gas system length (bolt face to mid point of the gas port) is the barrel length minus the dwell length minus half the diameter of the pin gauge.
This is a lot of effort to collect an empirical measurement. Assuming you are using a gas system length that conforms with industry standards, you can calculate the true gas system length.
The nominal gas system length is associated with the distance between the front of the barrel flange and the gas port.
The distance between the bolt face and the front of the barrel extension flange is consistently 0.8″.
To calculate the bore distance between the bolt face and the gas port, simply add 0.8″ to the length associated with the nominal gas system length.
The table below provides the flange-to-port and bolt face-to-port distances for the common nominal gas system lengths.
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