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Carbine vs. Mid vs. Rifle: A Beginner’s Guide to AR15 Gas Systems

Learning How AR 15 Gas System Works

Are you planning on buying or building an AR-15? Or changing out parts such as barrels, gas blocks, or handguards? If so, it’s important to understand how the gas system on an AR-15 works, why it’s so important, and how the length of your gas system can affect the performance of your rifle.

To make it simple, if you have the wrong length of gas system for your barrel, it can cause your gun to not cycle properly, or wear itself out too quickly.

So What Exactly is a Gas System?

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” gas system.

The below picture from Wikipedia demonstrates how the gas travels:

How gas travels in AR-15

 

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 not cycle or may jam.

 

US Army Service Manual AR 15 Ebook

 

What is the Correct Gas System Length for my Barrel?

The below chart shows the typical gas system lengths (distance from receiver to gas port) and how they pertain to barrel length. As you can see there are wide ranges here, typically the closer that the barrel length is to the middle of the range, the better it will function (example 14” barrel with a carbine length 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 20 plus inches 12 inches

Other factors

It’s worth mentioning that there are factors that can affect the gas system, beyond the length of the system.

Adjustable gas blocks – some gas blocks 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 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 heaver 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.

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.

28 thoughts on “Carbine vs. Mid vs. Rifle: A Beginner’s Guide to AR15 Gas Systems

  1. i have a lower receiver with carbine length buffer tube assembly.
    i want to use and upper receiver with a 20″ barrel. My question is, will the carbine buffer assembly cycle properly with a rifle length gas system?

  2. Jeremiah,

    That should work just fine. You aren’t going to ruin anything by putting a rifle length upper on you carbine lower.

  3. The carbine and rifle bolt carriers are not the same. I have a lower receiver with the shorter buffer tube and buffer and a 16″ upper and 20″ upper. There are no issues with cycling on either, but the bolt carrier on the 20″ upper will not lock back when the magazine is empty. I still have to contact the manufacturer to find out if it can cause any damage. I was trying to do the same thing and only have one lower receiver, but I will probably start building a rifle lower receiver soon.

  4. If I use a 10″ barrel to build an AR15 rifle what size of gas tube will I need?

  5. Your bolt carrier group not locking to the rear when the magazine is empty may be caused by a worn or low quality follower in your magazine. After the last round is fired, the magazine spring should push the follower to the top of the magazine. At the same time the tab on the bolt catch should contact the back of the follower and be pushed upward. As soon as the round is fired and the bolt carrier group moves to the rear, the bolt catch should be in the “up” position. This will keep the bolt carrier group locked in an open position.

    Worn or poor quality followers can fail to push the bolt catch up, causing this problem. I suggest upgrading your followers with Magpul followers, it will make your rifle more reliable.

  6. John,
    Wrong answer, Bolt carriers are the same length, There are 2 profiles semi auto and full auto. The semi has a shorter bottom extension at the rear and is therefore lighter than a full auto. My semi auto in my Bushmaster varmit rig is the same length as my BCM full auto carrier.

  7. I just built a mid rifle but used a carbine buffer. What I’m seeing is the bolt carrier does not fully retract and once in a while it will not cycle the next round and it does not Lock open when empty. Any thoughts?

  8. Rifle length gas system 18″ to 20″ barrels

    I use a Carbine lower M4 Style stock

    My Buffer is a H6 Damaged Ind. Buffer (5.2 OZ) and Enhanced Carbine CS spring on my MK 12 with a LMT Sopmod stock rifle length gas system

  9. hello I am trying to put a low profile piston operated system on my Smith and Wesson AR 15 carbine linkth rail and a 15 inch free floating rail after I install the piston kit what kind of piston kit do you recommend thank you

  10. I just bought a 14 inch rail for my ar 15, it’s a 16 inch rifle. What do I need to buy to be able to put the rail on? And what steps to take to put the gas block on? What gas block? Or any other items I must buy to put the rail on and change the gas system

  11. I purchased an 11″ barrel for an AR pistol build only to find there was no gas port drilled. Do you know the diameter of the gas port and what are your thoughts on installing an adjustable gas block? Thank you.

  12. I just bought an Adams arms rifle length 16.5″ bbl what length of hand guards do I need
    ?

  13. what do I need to change my carbine length AR to rifle length?

  14. I’ve got the carbine 15. What do I need to change my 7″ handguard into a 10″ handguard tube?

  15. I am wanting to put a long quad rail on my dpms oracle, it has the 6.5 handgaurd that is now an mfg quad but my accessories are getting a bit snug.

    My question is, i have the large gas block with the rail on top and want to know if the quad rails from say midwest or troy with the cut out for a front sight will go aound the gas block rail? Is it a waste of time? Should i just mount a low pro block and full float and entended to cover it?

  16. Building my first ar15 firing 6.5 Grendel. I have a 20″ Shilen match barrel and need to know what length gas tube and hand guard to use. I have a 15″ currently.

  17. Who cares, and no it won’t cycle with the carbine buffer and rifle upper. You need a H-14.644 buffer from the mini 30

  18. tube length for a DPMS LR308 upper with a 16 inch DPMS barrel?

  19. Hello,
    I have a 16″ 300AAC rifle. I’m purchasing a suppressor and handload my ammo. I’ve got the loads down to subsonic (around 950 fps). I want to file a form 1 and cut my barrel to 14. How will this affect the gas flow/cycle with using the subs with such little powder. Thank you

  20. I have a new build ar15 using m4 setup but a 20″ barrel with a rifle length gas system. I have a failure to extract. The bolt doesnt come back far enough to unchamber the spent cartridge. I have ruled out the BCG. The std spring and buffer are used and ive checked the position of the gas block. What could be the problem??

  21. i have a colt LE6920 with a 16 inch barrel. i am looking to shorten barrel length to between 11-14 inches. what is the best mil spec barrel to buy and and what concerns related to the gas chamber

    1. BCM makes a nice 11.5″ upper assembly, or just the barrel. Mine has worked 100%.

  22. I have a 16″ barreled 6.5 Grendel upper, which I bought an 18″ 1:8″ twist barrel to replace the original for the added velocity. The port on the new barrel is positioned for a mid-length gas system; is this going to be hard to tune in comparison with the original system? Any advice would be helpful. Thanks in advance!

  23. I want to switch my bushmaster varminter from direct impingement to piston I would like to know what the length of the gas system is on the varminter

  24. I would like to challenge one of your measurements. For the A1/A2 handguards to fit you need at least 12 & 1/8th inches from the front of the barrel extension flange to the shoulder on the barrel for the front hand guard cap/back of gas block. Add another .13 inches for the flange it self and another.38 inches from the shoulder to the port center and the distance from front of receiver to gas port should be closer to 12.435 inches.

  25. Great article. I would suggest adding a section on the use of suppressors and how they affect timing and gas pressure.

    1. Hi Mike,

      Thanks for taking the timeout to read our articles. We will look into adding a section on the use of suppressors. Until then, we will continue to produce content that would benefit AR15 owners.

      All the best,
      Alvin

  26. One thing I notice is most people do not try the mid length uppers with an A2 lower. I have built three of them using the rifle receiver and rifle buffer and they work very smooth. The same uppers on a carbine lower work just fine.

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