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All About AR-15 Gas Tube Lengths

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Aaron Spuler is a firearms enthusiast and recreational shooter. Follow more or his work at The Weapon Blog

The gas tube in an upper receiver is one of the more essential features of an AR15 rifle that we often overlook. While veteran DIY budget builders understand how a direct impingent gas system works, the entry-level owner attempting their first AR15 build needs to have a basic understanding of the importance of the technology.

How the Gas Tube System Works

Every semi-automatic rifle has a gas port that vents a small portion of hot gas created by the explosion of the primer and propellant inside the shell. The vent acts as a release mechanism, much like when you vent steam from a pressure cooker.

However, the gas also goes up through the gas block, then travels through the gas tube. It then contacts the bolt carrier group which cycles back and allows for semi-automatic function. The gas tube and the bolt carrier group are where the magic happens because the stream of hot gas auto cycles the bolt carrier group that controls the ejection and insertion of your ammunition.

Mismatching the Gas System to the Barrel

The technology is straightforward and typically functions as expected unless you mismatch the gas tube length with the associated barrel. Pairing the wrong size of a gas tube with the barrel you install can result in less than anticipated results.

To understand what length the gas tube needs to be for the barrel you’ve chosen, you need to know how the direct impingement system is supposed to function. One rule of thumb is important to remember. The length of the gas tube to the port should always increase as the length of the barrel does.

Dwell Time

You may not have heard of the term “dwell time,” but it’s crucial when choosing the size of your gas tube. Dwell time is how long the projectile remains in the barrel after passing the gas port. Although it’s a matter of microseconds, there are a lot of things happening simultaneously. The longer the barrel, the more microseconds the bullet remains in the barrel even after it passes the gas port.  

The gas tube conveys the gas necessary to operate the auto-cycle functionality of your AR15. If the tube installed is too short, which means a disproportionate length of the barrel to the gas tube, you will probably experience a recoil that’s stronger than getting kicked by a mule. The excessive wear and tear of firing your AR15 under these conditions will guarantee quicker failures and much more maintenance time than you’ll probably want.

Conversely, if the gas tube installed is too long, meaning that it’s again disproportionate to the length of the barrel, insufficient gas will travel to the receiver. The result will almost always be a jam or failed cycle of the next round. However, if you are building a short barrel rifle with an 11.5” barrel, then either a carbine length or mid-length gas system will work.

What Size Gas Tube System is Needed

By now, you’re wondering how you’re supposed to know what length of gas tube you need for your AR15 build. Remember, the size of a gas tube is the length from the receiver to the gas port. Instead of whipping out your ruler and measuring your build, use the following guidelines.

For a carbine where the barrel is between ten and eighteen inches, you’ll need a gas system that is seven inches long from the receiver to the gas port. If you’re building a mid-length AR15 which is popular these days, the gas tube system needs to be nine inches in length. Any rifle built with a barrel that’s twenty inches or longer requires a gas tube system that’s twelve inches long from the receiver to the port.

Learning the basics of installing the proper length gas tube system is simple if you apply these common-sense rules and suggestions. Still, there are a few other things that you’ll probably need to understand about a direct impingement-operated rifle.

Other Things You’ll Need to Consider

A few things can and will affect how your AR15 will operate regardless of whether you’ve installed the correct size gas tube, such as an adjustable gas block. With it, it’s entirely possible to control the flow of gas so that you can reduce recoil.

Another thing that could affect the way your AR-15 handles is a heavier than usual buffer.

Think of it this way. The gas traveling through the tube system must move the buffer to cycle a spent shell out and another round into the chamber. The heavier the buffer, the more gas will be necessary to move it. 

Having a heavier buffer is not always a bad thing, though. Heavier buffers are frequently used to moderate a rush of excessive gas through the system and can help smooth out the ammunition cycling process.  

Additionally, if you’re firing low-power ammunition, each shot may not produce a sufficient amount of gases to cycle the rifle properly. Make it a rule of thumb to utilize the gun properly by firing the ammunition intended, and you should be okay.

Many DIY AR15 builders who’ve been around the block more than once typically choose the correct gas tube system for the length of the barrel they’ve chosen. Unfortunately, just as many had to learn the hard way. Many of these veteran builders probably ended up with frustrating jams and failures to cycle when first building their rifles.

To lessen the pain of a bad day attempting to shoot a rifle that fails at the drop of a hat, consider making the selection of the proper size of gas tube system just as important as your choice of triggers, stocks, and barrels.  

When you do, you can guarantee that you’ll have many pleasurable hunting expeditions and exciting trips to the range.


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