So, you witness a debate with some friends on which length gas system is best for the and you think to yourself “huh, what’s the difference?”. We did a real deep dive for you this time. Why is your AR-15 gas system choice so important? In short, if you have the wrong length of gas system for your barrel, it can cause malfunctions or excessive wear. Stick with us and we will break everything down below!
Table of contents
What Gas System Does The AR-15 Use?
Let’s start by asking Wikipedia:
“Gas-operation is a system of operation used to provide energy to operate autoloading firearms. In gas-operation, a portion of high pressure gas from the cartridge being fired is used to power a mechanism to extract the spent case and chamber a new cartridge.”
So on an AR-15 specifically, there is a small “gas port” (small hole) in the barrel that vents gas with every shot. This gas travels through the gas block and gas tube into the receiver, where it powers the bolt carrier group and auto-cycles the next round. This is known as a “direct impingement” system.
The GIF below from Wikipedia demonstrates how the gas travels:
Why Does the Length of the Gas System Matter?
The length of the gas system (length from the receiver to the gas port) should increase as the barrel length increases. The reason has to do with “dwell time” – the length of time that the bullet is in the barrel after the shot is fired.
On a longer barrel, the dwell time will be longer, because the bullet has to travel a longer distance. Specifically, there is a tiny amount of time when the bullet is traveling through the barrel and it is past the gas port, but it’s still in the barrel. During this time, there is hot gas traveling through the gas tube into the receiver. As soon as the bullet leaves the muzzle, the gas stops flowing.
If there is too much barrel length after the gas port, then too much gas flows into the receiver and it can causes issues with excessive recoil and wear on the rifle.
If there is not enough barrel length after the gas port, then too little gas flows into the receiver and the rifle may malfunction.
What is the Correct Gas System Length for my Barrel?
The chart below shows the typical gas system lengths (distance from the receiver to the gas port) and how they pertain to barrel length. As you can see there are wide ranges here, typically the closer the barrel length is to the middle of the range, the better it will function (for example 14” barrel with a carbine-length gas system). Something to remember is that the right gas length is a set value and it is highly dependent on the barrel being used. We highly recommend looking at the barrel manufacturer’s specifications regarding their suggested gas system.
|System||Barrel Length||Port Distance|
|Pistol||Less than 10 inches||4 inches|
|Carbine||10-18 inches||7 inches|
|Mid||14-20 inches||9 inches|
|Rifle||18 inches or more||12 inches|
AR15 Gas System Considerations
It’s worth mentioning that there are factors that can affect the gas system, beyond the length of the system. If you have reoccurring issues and you suspect your gas system is to blame, check out page 110 in this FREE US Army Manual for the AR15 / M4 / M16.
Adjustable gas blocks – Some gas blocks are available that allow you to throttle down the amount of gas that travels through them. For example, you can constrict the gas to the point that the rifle is a single shot.
Buffer weight – The weight of the buffer can have a big effect on how the AR15 cycles. A heavier buffer will generally require more gas pressure to cycle, so it is possible to moderate the effect of too much gas pressure and smooth out the action by using a heavier buffer, or vice versa with a lighter buffer. Buffer springs can also have an effect.
Ammo type – The weight of the bullet and the powder in the cartridge can make a big difference. A heavier bullet may travel more slowly and cause a longer dwell time, whereas a low-power cartridge may not build enough gas pressure to cycle the action properly.
What is a Piston System?
While you poke around on the internet looking for answers (we got em) you may stumble across the piston gas system that has made a resurgence in the past couple of decades. Piston systems operate on similar principles as direct impingement in that there is a hole in the barrel that vents gas into a tube. The big difference is that inside this tube is a piston that either strikes a bolt carrier group (BCG) or is attached to one. Notable firearms that use this would be the M1 Carbine, M14, and AK47.
The GIF below from Wikipedia demonstrates how the Short Stroke Piston Gas System Works:
Piston-driven systems (long stroke and short stroke) are nothing new and have their apparent attributes such as not relying on a buffer system to the rear. This eliminates the need for a fixed extension of the lower receiver.
Their critics will make sure to note that these types of systems are not only heavier on average but also the system is not standardized. This means companies who make guns with this system or parts for this system, will have proprietary designs. Nothing will really interchange as far as the gas system is concerned.
Ideally, we will cover this topic separately at a different time. We just wanted to hopefully sever any confusion about its lack of in-depth inclusion.
What Length Gas System is Better? Mid Length vs Rifle Length vs Carbine Length Gas System
The most agreed-upon consensus is the longer the gas system the better overall. The main reasons for this would be that there is less wasted powder, velocity, and energy. More positive attributes include a smoother cycling feel and lighter felt recoil. Even though carbine length is more common off the shelf (as far as stock guns) it is accepted that both mid-length and rifle length are very comfortable in comparison to their shorter counterparts.
Do those things make the mid-length and rifle length better? No, not necessarily.
Carbine length is more common on shelves because it is not only a shorter more maneuverable option but it is a sort of happy medium. It’s versatile! It fills most roles to an acceptable degree. Going with a shorter or longer gun would lessen the roles it could easily fill. In short, carbine length is a do-all.
“Better” is relative. Do you want a more comfortable and efficient shooting experience? Maybe go with the longer options. Do you want a close-quarters home defense gun? Try the pistol length. Do you want one and done? Roll with the old faithful carbine length. Pick which one is best for your intended role and you will never look back thinking you should have got something “better”.
One Last Tip
If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.