Ask Foghorn: Gas System Length on the AR-15

Tyler Kee asks:

Carbine, mid length, rifle length. What do they mean? Can I run a carbine length on an 18″ barrel? A rifle length on a 16″?

What gives?

Well, you can do whatever you want, but that doesn’t mean the gun will run. Choosing the right gas system is a delicate balance between getting enough gas to cycle reliably and keeping your rifle from beating itself to death. So, which one is right for you? And what is it all about? . . .

Hiram Maxim figured it out first — there’s a TON of wasted energy when a gun goes off, energy in the form of recoil and noise. And if you could capture that energy, you could use it to do something useful like cycling the firearm automatically. Maxim’s design used the recoil of the firearm to reset the gun and load the next round, but it required a massive gun, and wasn’t exactly mobile.

The Vickers gun, with its “muzzle booster,” was one of the first designs to use the force of the expanding gasses rather than the force of the recoil to work the action of the firearm. Similar firearms, such as the M1 Garand and G41(M) would continue to use devices attached to the muzzle to redirect gasses to operate the action, leaving the barrel completely in tact. Browning took it one step further with the B.A.R. or Browning Automatic Rifle, which used a gas system that we would recognize today.

The standard modern gas system (illustrated here thanks to Wikipedia) uses a small hole in the barrel much before the muzzle to redirect some of the gasses to work the action. Bleeding off the gas before the bullet leaves the barrel means the gasses will be MUCH higher pressure than they would be at the muzzle, and allows the action to cycle faster.

There’s a slight issue with using a gas port, especially on the AR-15 platform: barrel length.

In order for there to be enough force to cycle the action, the bullet needs to still be in the barrel when the gas hits the bolt carrier, like a cork in the champagne bottle keeping the pressure up. This is also referred to as “dwell time,” the measure of how long the bullet is in the barrel.

And since the bullet is constantly moving, there needs to be a sufficient length of barrel left so that the bullet leaves the muzzle only after there has been enough gas transferred to the bolt carrier to make the whole thing work. Too much barrel and the bolt is “over gassed,” leaking all over the place making a mess and possibly damaging the system by applying too much force. Too little barrel and there isn’t enough force to unlock the bolt and fully cycle the action.

The original AR-15 came in one barrel length, 20 inches. But as we have shortened the barrel over the years the gas system has needed to be adjusted accordingly to keep the right balance between giving the gun enough gas to cycle the action reliably and keeping the gun from tearing itself apart, since too much rearward force can damage the buffer assembly and other components.

The trick is, as you shorten the barrel, you should move the gas port closer to the muzzle in order to decrease dwell time after the round has passed the gas port. For example, the difference in pressure 9 inches out and 7 inches out from the chamber is almost double, meaning the bullet should probably leave the barrel sooner with the 7 inch port. There are some exceptions to the rule, but for the “proper” gas length that’s the idea.

In order to make things slightly simpler for us consumers, manufacturers have standardized the length of gas systems and provide a guide for which system goes on which length barrel. Here’s ye olde chart:

System Barrel Length Port Distance
Pistol < 10 inches 4 inches
Carbine 10-18 inches 7 inches
Mid 14-20 inches 9 inches
Rifle 20+ inches 12 inches

You can see that the distance between the port and the minimum barrel length increases as you go up the chart – carbine is 3 inches, mid is 5 inches, and rifle is 8 inches. Which keeps things under control.

The problem is with the carbine length gas system. People love to have their stuff “mil spec,” and since the military spec is a 12 to 14 inch barrel a carbine gas system is what they use. Unfortunately, with that whole National Firearms Act thing, civilians generally are restricted to 16 inch barrels. Using a carbine gas system on a 16 inch barrel will work (and work remarkably reliably), but you run the risk of having your rifle wear out quicker. It also leads to some nasty backpressure when using a silencer, which tends to leak out the back of the receiver and straight up your nose. Ask me how I know.

The main exception to the rule is weird calibers. With 5.56x45mm NATO the chart is perfect, but when you have things like .300 AAC Blackout that have different pressures and such it gets complicated. 300BLK is recommended to have a carbine length system for anything 16+ inches and a pistol length system for anything under 16 due to the powder being used, but in general you should listen to the manufacturer’s recommendation for what system to use.

And that, Mr. Kee, is what its all about.

[Email your firearms-related questions to “Ask Foghorn” via guntruth@me.com. Click here to browse previous posts]

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About Nick Leghorn

Nick Leghorn is a gun nerd living and working in San Antonio, Texas. In his free time, he's a competition shooter (USPSA, 3-gun and NRA High Power), aspiring pilot, and enjoys mixing statistics and science with firearms. Now on sale: Getting Started with Firearms by yours truly!

15 Responses to Ask Foghorn: Gas System Length on the AR-15

  1. avatarJason says:

    You didn’t mention one of (if not the) most important factors: port size. Gas = time x pressure x port area. The shorter your barrel after the port, the more critical a properly drilled port becomes. Unless you’re making your own barrels, it shouldn’t really be an issue: the makers know what diameter port to use for a given cartridge and length (both barrel and port position), and a quality manufacturer will drill it with precision. Even 10.3″ barrels with carbine-length systems can be made to work. Pick the length(s) you want, buy it from a decent company, and stop worrying.

  2. avatarAaron says:

    I run a dissipator (16″ bbl with a rifle gas system and a larger port). You’ll never shoot a softer or flatter recoiling rifle in your life. I have an adjustable gas block, but it runs just fine without it.

  3. avatarfrankgon4 says:

    Great, I have an AR in 7.62 X 39
    Wonder how this will work out in the long run.

  4. avatarheadless says:

    Do yourself a favor.. scrap that stupid anatomically incorrect diagram.

  5. avatar16V says:

    What in the wide, wide, world of shooting sports is this article talking about? I have no idea how I found this old post, but egads…

    The standard modern gas system (illustrated here thanks to Wikipedia) uses a small hole in the barrel much before the muzzle to redirect some of the gasses to work the action.

    HUH? That’s the illustration for a “direct impingement” gun (the AR/M) not the other illustration for every other gas op made on planet earth. Every “modern standard” gas-op rifle uses the gas piston illustration – if one were to go to wiki.

    There is only one, ONLY ONE gun currently in production that is direct impingement. ONE. The AR/M. Nobody else has even had a direct impingement rifle in arsenal for 50 years, let alone actually made one. Nobody designs them, puking in your mouth to feed yourself is inherently flawed in at least 4 ways, and every gun designer on the planet knows that. That’s why nobody does it. Nobody.

    Direct impingement is also why the AR/M so incredibly load sensitive.

    Every single other gas-op battle rifle currently in service (as well the last 70+ years) has a GAS PISTON. The Vickers machine gun, M1 Garand, BAR, the AK47 and every battle rifle in this article, not to mention currently made on the planet that is gas operated has a gas piston which moves a rod, which the cycles the action.

    The G41 mentioned uses a completely opposite system of using exhaust gasses to pull on a rod to cycle the action. How did that even get in here?

    The only direct impingement rifle that has been made in the last 60+ years in the AR/M. That’s it. It is the only gun that directly channels exhaust gas into it’s receiver to operate the action. The ONLY one.

    Out of the hundreds of gas op rifles designed and built over the last 80+ years, only 3 have been built that don’t have a gas piston. Only 3. MAS 40, Ag m42 and the AR/M.

    • avatarMax says:

      And yet there are millions of them in service throughout the world, and more sold every day…

    • avatarSouthern Cross says:

      And the two offshoots of the AG 42, the Hakim (AG 42 in 7.92×57) and the Rashad (AG 42 in 7.62×39). The AR-15′s usage of direct gas impingement was to reduce the weight of the firearm and to improve accuracy without so much metal hanging off the barrel.

      I remember a rule there had to be about 4″ of barrel after the gas port to provide adequate gas pressure in a AR type design. Can anyone elaborate on this?

    • avatarMax says:

      I’m not an internet commando by any means, but even I know that accuracy international recently made a .50BMG semiautomatic rifle that used direct impingement, and called it the “AS50.” China also actively fields a direct impingement driven semiautomatic rifle in 12.7mmRussian/.50BMG. The rebels in Syria and Burma they’re supplying it to supposedly love it, and I know the Syrians at least have a number of Barretts to compare them to.

      Knowing of at least 2 new rifles just off the top of my head that are DI, 1 in active service and production, I know you’re full of shit. The writer obviously meant the method of bleeding off gas was a small port in the barrel, and was not trying to say most systems are DI. Get your shit together, brony.

  6. avatarLee Murphy says:

    Contrary to popular misconception, the Stoner & Sullivan design in the AR is a gas piston system, not direct impingement. The rear of the bolt is the piston and the front of the carrier is the cylinder.
    There have been very few true DI guns with none currently in production that I’m aware of.
    No such thing as a free lunch and all that, which op system is best depends on what you are using the rifle for and how you’re doing so. For me, the Stoner & Sullivan design is fine.

    Thanks,
    Lee

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  8. avatarRevive says:

    I own several other firearms aside from the AR platform, and am considering adding one to the safe – though I’ve had the pleasure of firing both DI and GP versions.

    Since I’m entirely underwhelmed by the general performance and value of the .223/5.56 NATO round, and don’t prefer to get beat to a pulp with an AR-10 or other .30 cal rifles, my goal is to procure (or build) an AR in 7.62×39 – I know, I know.. the commie round, thought to be such a poor choice. Especially when fired rifles in the hands on inexperienced shooters, farmers, etc. But, I like the round. I like the ballistics and I rarely will engage a target beyond 250 yards (target or dinner). The price is also a helpful factor because I love to shoot! Good choice @frankgon4

    So, here is the question/request for input:

    I’d like to build/buy an AR in 7.62 with a 16″ barrel, rifle length gas system and enlarged gas port (with an adjustable gas block) – much like @Aaron’s above .
    I don’t want/need Bolt Assist and will definitely go with a free floated hand guard and A3 type upper receiver. I already have the stock/furniture choices made, most of which is Magpul.. so what I’m really looking for is suggestions on the upper/lower or complete rifles.

    So,…. suggestions on where to buy or which parts/components to use? What is your experience with this cartridge in the AR platform? If you’ve had issues with this cartridge have you addressed your feed ramps, got the correct magazine, etc?

    Thanks again for such a great blog and look forward to hearing your feedback!

    • avatarRevive says:

      – one quick addition to my post above. Depending on whether I decide to scratch this itch, I would also want the AR i build/buy to be compatible with other uppers (eg. I won’t be modding the AR to take an AK mag, etc.) One such upper I’ve been eyeing, even though it’s totally impractical – and simply awesome – is a .50 Beowulf upper (plus proper buffer and spring to calm it down a bit).. there it is.

  9. avatarScotty says:

    Hey Revive, I did the 7.62×39 AR thing myself and I’m pleased as peach. I purchased a complete lower from Surplus Arms and a complete barreled upper from barrier defense. This will get you a complete AR for less than $700.00 total with a free floating hand guard and 16″ heavy barrel. I bought a 30 round C-products mag and so far it has all ran flawlessly. I hope this helps you out and saves you from having to mod mags and such. Good luck. Oh, yeah I got a bunch of wolf ammo I haven’t tested yet, but this thing really loves the steel cased hornady SST. 1.5″ three shot groups and I’m no sniper.

    • avatarRevive says:

      @Scotty,

      Thanks for the reply! I’ve been toiling over whether to buy a complete rifle, buy separate upper/lowers or build the lower and buy the upper.. Sounds like you went with the least painful and simplest solution :D

      What length gas system is your running? Not sure if I want to keep with the std 16″ barrel, would love to build an SBR, but not sure if the x39 round would breath too much fire out of a 14.5″ or 12.5″ barrel. From what I read, the FPS don’t suffer much between the different lengths and since 7.62×39 doesn’t depend so much on velocity (not as much as .223 does), it doesn’t bother me to run 20-40 FPS slower than a 16″ barrel.
      Thoughts?

  10. avatarPaul J Schonbrun DO says:

    Funny how the dissipator is known as a good gun and the new Salient Arms’ 16″ carbine has a rifle length gas system and is reported to be the ‘cat’s ass’ according to the Gun-rag meatheads. Amazing, how Pat Rogers has AR’s that are rack guns at his school that have NEVER BEEN CLEANED with 19,000 rounds through them…he just lubricates them. When AR’s don’t work, it is primarily due to magazine/follower issues, extractor issues (Yes, those little donuts really do help) and crappy steel case ammo…and having said that, I have an old M15A1-model 601 and an M15 model 607 carbine rebuild that will eat Wolff ammo all day long!!! My 10″ SBR is a flawless, fire breathing dragon with a pistol length gas system…Why, because the ammo, the short gas system allows adequate dwell time in the barrel allowing the bot enough time to unlock, extract eject, reload and relock while the extractor and the recoil system are in simpatico. The AR system is built on a balance of gas impulse, extraction forces and recoil and buffer spring balance. I’ll take my mid-length DI carbine with an extractor donut, a Horus talon scope and a giessele trigger and hit steel targets at 600 yards all day long. The AR might be a dirty rifle and doesn’t like sand; but there aren’t many that do. The trick is just a little bit of oil. My ArmaLite match service rifle M15A2NM with iron sights will hit the grapefruit sized X ring at 600 yards (if the wind is blowing right and I do my part) all day long. If Salient Arms (whose rifles and pistols are the real stars of the new ‘Transformer’ Movie) and the old Dissipator series carbine, which has been a media darling for years, then they must be doing something right! I’m building a rifle length gas system on a 16″ barrel and an adjustable gas port on for my next Upper receiver build. I can’t afford the king’s ransom which Salient Arms is asking for their rifle, but I can sure as hell borrow their research and technology to build my own!

    As far as Mr 16V poo-poo’ing the AR15, America’s longest serving military long arm, I can only say “blow it out your gas port, windbag”.

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