Complete Guide to AR-15 Gas Systems
Whether you go with a gas piston conversion or a traditional gas tube for your AR-15, deciding what gas system length is the right option for you can be a confusing process. With several options to choose from, in seemingly limitless barrel configurations, it’s easy to get lost. Today we’ll be looking at the most common lengths for AR-15 gas systems and we’ll cover the pros and cons of each to help you make an informed decision
Gas system length and effects don’t change between Direct Impingement or piston-driven.
What Does the Gas System Do?
The AR-15 is a gas operated firearm with a multi-lug rotating bolt that fires from a locked breach. Let’s break this down. When firing, the bolt is locked into the barrel extension. To operate and cycle the action, the rifle will use some of the high-energy gas that propels the bullet down the barrel to push the bolt carrier, unlock the bolt, and extract the empty casing. As the bolt carrier group moves rearward and the casing is ejected, the buffer and buffer buffer spring inside the stock assembly will absorb the energy and push the bolt and carrier back forward. As this happens, the carrier strips a new round from the magazine, feeding it into the chamber, and then locking the bolt lugs back into the barrel extension so the rifle is ready to fire again.
With a standard AR-15 the gas that makes this all happen is fed through the gas port (a small hole drilled into the top of the barrel), into the gas block, and then down the gas tube. The gas tube enters the upper receiver and feeds into the gas key on the bolt carrier. Inside the bolt carrier group, that gas fills a small piston chamber inside of the bolt carrier formed by the tail of the bolt, which pushes the carrier backwards. This causes the bolt to rotate and unlock, allowing the action to cycle again. Alternatively, a gas piston-driven rifle feeds gas through the same gas port and gas block arrangement as a standard AR-15, but instead of pushing the gas through the gas tube to cycle the bolt, it pushes an external piston instead, and that piston’s displacement moves the bolt carrier to cycle the rifle. For this article, we are referencing the more common direct impingement system with a gas tube, but the information on gas system lengths and their advantages or weaknesses will hold true for both options
Pick Your System
Whether you want to use a gas piston or a traditional direct impingement gas system for your AR-15, it is important to know the “why” behind the different gas system lengths. Today we’ll be looking at the four most common gas system lengths you’ll find, how to identify them, and how they can meet the needs of your build. Longest to shortest, they are: Rifle, Mid-length, Carbine, and Pistol. The longer the system, the further away from the receiver the gas port and gas block will be located down the barrel. Where the gas port is located has a drastic effect on a rifle’s reliability and felt recoil, as well as long-term parts durability. This is because more or less gas pressure is available to cycle the rifle depending on the length of system. For the purposes of this article, we’ll be treating these gas systems as ‘all else being equal’ and ignoring the various buffer weights and systems, which also influence recoil, reliability, and parts durability. Before we get into it, let’s take a quick look at a few terms that are going to pop up going forward:
Gas Port Length: Distance from the face of the upper receiver to the center of the gas port
Gas Tube Length: Length of gas tube (for standard direct impingement AR-15s). This is a rough measurement; some minor variance is expected and normal.
Dwell Time: Dwell Time is amount of time the bullet spends in the barrel after it has passed the gas port. Too much dwell time and your system could be over-gassed, too little and your rifle may not cylce properly.
Now… let’s get to it
Rifle Length Gas System
- Gas Port Length: ~12”
- Gas Tube Length: ~15”
- Ideal barrel length: 18”+
The rifle length gas system places the gas port roughly a foot down the barrel. This placement allows the bullet to travel most of the way down the barrel before it passes the gas port and gas can be siphoned from behind it, offering a longer dwell time and providing an opportunity for better mechanical accuracy. By extending the dwell time, pressure within the chamber and barrel are reduced as well. Lower gas pressure means the bolt carrier group will move more slowly translating to lower felt recoil. This lower pressure and reduced rearward bolt velocity also put less stress on the internal components of the firearm than other gas systems, theoretically increasing the life of many small parts.
This reduced gas pressure does have its downsides, however. With less energy to cycle the rifle, it may be more sensitive to dirt and fouling, requiring more regular maintenance to keep it running optimally. Rifle length gas systems also work best when used with longer barrels, so isn’t well suited to compact builds.
- Reduced felt recoil
- Reduced wear and tear
- Better potential mechanical accuracy
- Reduced adverse reliability
- More ammunition sensitivity
- Not suited for short or medium length barrels or compact builds
Mid-Length Gas System
- Gas Port Length: ~9”
- Gas Tube Length: ~11.75”
- Ideal barrel length: 14.5″-20″
The mid-length gas system places the gas port roughly 9” down the barrel, providing a nice compromise between a carbine length gas system’s reliability and harsh cycling, and rifle length gas system’s smooth operation. Mid-length gas systems are extremely popular for their generally smooth recoil and reliability. It is not the optimal choice for a proper precision rifle, nor is it the optimal choice for short barreled firearms but it is great for 16” or even 14.5” builds.
- Less recoil than Carbine gas systems
- Less parts wear than Carbine gas systems
- Less sensitive to dirt and fouling than Rifle gas systems
- Reduced reliability in adverse conditions than Carbine gas systems
- More parts wear than rifle length gas systems
- More felt recoil than rifle length gas systems
Carbine Length Gas System
- Gas Port Length: ~7″
- Gas Tube Length: ~9.75”
- Ideal barrel length: 10.5″-16″
With the gas port placed 7” down the barrel, there is very little time for gas pressure to be reduced before the bullet passes the gas port. With more high-pressure gas available, the bolt carrier group can be cycled with greater authority improving the rifle’s ability to function properly even when it is extremely dirty. This reduced dwell time does have its drawbacks, of course. The more violent action of the carbine gas system translates to a sharper recoil than the above options, as well as a slight increase in the wear and tear of small parts.
- Most reliable options in adverse conditions
- Least ammunition sensitive
- Ideal for shorter and medium length barrels
- Increased wear and tear
- Increased felt recoil
- Reduced potential mechanical accuracy
Pistol Length Gas System
- Gas Port Length: ~4″
- Gas Tube Length: ~6.75”
- Ideal barrel length: Less than 10.5″-16″
Sometimes a barrel is simply too short to support a more traditional gas system length and for those times there is the pistol length gas system. This gas system is typically found on AR pistols or registered short barreled rifles with 10.5-inch barrels or shorter, though many rifles chambered in 300 BLK will use the short pistol length gas system to maximize reliability with subsonic ammunition, even on barrels as long as 16”. The short dwell time and high pressure allow the firearm to function reliably with the short barrel but tapping the hot gasses so close to the chamber will lead to increased fouling, higher operating temperatures, and increased wear over carbine length systems.
- Compatible with very short barrels
- Reliable performance with subsonic 300 BLK
- Increased wear and tear
- Increased fouling
- Not suited for longer barrels
Now, of course there is more to the story than the gas system. Felt recoil can be drastically altered with the use of muzzle brakes or using lighter or heavier buffers. Adjustable gas blocks can fine-tune your rifle’s gas system to find the perfect balance of reliable function while minimizing felt recoil. To keep things simple though, we’re going to wrap up here. Simply put, every component you choose for your rifle is a compromise, and your gas system is no different. Choosing the perfect gas system for your build is simply finding the compromise that best suits what you want in your rifle. Besides, no one is saying you can only have one AR.
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