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.
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All the school shooters were liberal whackjobs
My local Gander Mountain is excellent. It has a live range, virtual range, and a simulator. Its concealed carry class instructors are very good. It has a large selection of firearms and ammunition. Service in the smaller gun stores in the area has improved since they now have to compete with Gander. It has been a long time since I’ve seen a gun club bore draped over the counter at any of them. That subspecies now seems to stick to gun shows in the area. Unfortunately, one of them is frequently either the cashier or hand stamper at those events.
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.
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.
Great, I have an AR in 7.62 X 39
Wonder how this will work out in the long run.
Do yourself a favor.. scrap that stupid anatomically incorrect diagram.
I would assume there is no chemical means–some kind of thread-locker–that would do the job?
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
I ran into a guy with a pistol from this manufacturer a few years ago. I held it. Tolerances were tight and it felt like a sold piece of boat anchor. I considered buying one because I don’t like the Deagle action too well. However, the proud owner claimed that the mags were over $200 and I refuse to buy a gun that needs that much for standard cap mags I could make myself if I decided I wanted 10 and had the time to learn and buy machinery to press my own spring fed ammunition devices.
The company struck me as boutique, like getting OFWGs to buy a gun worth bragging about after you have a stable of 1911s from every manufacturer. I see a point in the niche market. May Capitalism make simple decisions to the fate of the manufacturer.
The amount of double speak, delete key and explanations provided are only making murky what is a pretty clear point. As a result, the PR game being played here is good because the nature o of it makes intent far clearer. They want to tell us what we need to think about their message. If what we would come to isn’t good enough don’t beg for a redo or judging concessions; change the damn message.
If Rebecca is worried about polarizing any group when this manufacturer is not even as well known as fly-by-night companies selling guns in Shotgun News…it is like asking a 3 year old to contribute to a grown-up discussion. It’s cute, sounds nice but is wholly impractical given the size of the world compared to a toddler’s worldview. I could care less about Coonan before. Buying his stuff now seems sillier than ever but that wasn’t a hard idea to bring to mind.
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”.
So in other words, if the FSB is at near the very front as end using a rifle gas tube on a 16″ then that would cause the rifle to be undergassed?
What is the proper gas port hole diameter for a 16 inch 6.8 rem barrel. Plan to do this build myself. Got a barrel from sarco at a great price but needs the hole drilled. Or recommend someone to do it. Thanks c. jons
Great article and very helpful. I will be ordering a barrel this week and was unsure which way to go.
Thank you!
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I run a windham 16″ carbine length gas 7.62 x 39, auto carrier, std recoil spring, H2 buffer, .094 gas port, BCM upper, Windham bolt.
Due to the reduced chamber sealing with the steel cased ammo and increased bolt thrust, I believe more dwell time is needed for the bolt to push against the cartridge, come off the lugs, and rotate before the bolt is extracted by the carrier.
The gas pushes 2 ways: moves the bolt forward and the carrier rearward. Lighter bolt moves first. The bolts being “broken” or failing are being pulled through the locking lugs prematurely. A lot of gas with a very short dwell might do this.
Capice?
This is my theory and I’m stickin’ to it…:^)
I love my 7.62 x 39 ar. Everytime I shoot it, I can’t wipe the grin off my face. Runs like a banshee… Let’s see, 6.5# carbine at 25 cents a round with group sizes easily half my ak(s)…
Tom Prince aka Kivaari
I too really enjoy the 7.62×39 AR.
I have 4 and they all run great with both ASC and C-Products Defense magazines.