Mid-length vs. Carbine Gas Systems in AR-pattern Rifles
Both of these rifles have 16" barrels, but the gas system length (marked here by the position of the front sight tower) is different.
The M4 carbine has a 14.5" barrel and a ‘carbine’ gas system, meaning the gas port is at the 7.5" mark. Civilian clones of the M4 usually have 16" barrels for legal reasons, but traditionally keep the carbine gas system unchanged. However, ‘mid-length’ gas systems with the gas port at 9.5" are becoming more and more common, giving many civilian shooters one simple question: should I go mid-length or carbine?
If you have a 16" barrel and the cost is the same, the short answer is: get the mid-length.
The long answer is that there are six notable benefits of a mid-length over a carbine. The benefits are listed below in (roughly) the order of importance:
If you have a 16" barrel and the cost is the same, the short answer is: get the mid-length.
The long answer is that there are six notable benefits of a mid-length over a carbine. The benefits are listed below in (roughly) the order of importance:
Benefit #1: Healthier Bolt Velocity due to Lower Gas Port Pressure.
The further combustion gases travel from the chamber, the cooler they become, and cooler gases exert less pressure. Lower pressure equals a slower bolt velocity. One might think that higher pressure is better for reliability, since overdriving the action to slam the bolt backward will ensure successful extraction, but it’s not that simple. There is more than enough energy to extract even in cool-running rifle-length ARs, and overdriving an AR action to high bolt velocities will result in excessive bolt bounce and extractor slippage, magazine feed jams, and increased recoil. The goal is a “goldilocks” zone set by the original design (a 20” barrel with a 12.5” gas system); fast, but not too fast.
From the table below, we see that both mid-lengths and carbines are putting out substantially higher pressure at the gas port than a rifle. Have you ever noticed that the recoil of a carbine AR feels ‘snappier’ than that of a rifle? That’s the result of higher bolt velocity. M4 feed ramps and heavyweight buffers were also developed to accommodate the faster cyclic rate of carbines over the original, rifle-length design. However, of the carbine options, a mid-length is shown to be only about half as over-pressurized as a carbine. You can deal with overpressure by tuning and tweaking, but the mid-length offers a healthier baseline from which to build a reliable rifle.
From the table below, we see that both mid-lengths and carbines are putting out substantially higher pressure at the gas port than a rifle. Have you ever noticed that the recoil of a carbine AR feels ‘snappier’ than that of a rifle? That’s the result of higher bolt velocity. M4 feed ramps and heavyweight buffers were also developed to accommodate the faster cyclic rate of carbines over the original, rifle-length design. However, of the carbine options, a mid-length is shown to be only about half as over-pressurized as a carbine. You can deal with overpressure by tuning and tweaking, but the mid-length offers a healthier baseline from which to build a reliable rifle.
Benefit #2: Lower Gas Port Pressure Allows a Larger Gas Port.
To avoid overdriving the system, shortened gas systems usually use a smaller gas port to compensate for the higher pressure at the gas port. Cool-running rifle length systems have gas ports that are typically larger than 0.0935”, while hot-burning carbine length system range down to as low as 0.0625”. Mid-lengths are generally somewhere in between. The key here is that the margin for error is much higher with a smaller gas port; it’s better to hit the action with a big whallop of cool, low-pressure gas than a tiny spurt of hot, high-pressure gas.
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LuckyGunner.com conducted a study and found that the burn rate of powder across ammunition brands varies. One brand in particular utilized a powder that burned very fast, so fast that there was not very much pressure left by the time the gas reached the gas port. In a carbine AR with a small gas port, this ammunition performed extremely poorly, jamming so frequently that it had to be removed from the study. When the same ammunition was used in a mid-length AR, reliability improved to normal levels, even though the mid-length gas port was pulling gas that was even cooler and less pressurized than the gas (from the very same ammunition) that was short-stroking the carbine. This is because the mid-length's larger gas port was able to grab enough gas to capitalize on the what little pressure was available.
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From this, you might conclude that a big gaping hole of a gas port is the best of both worlds. However, remember that the carbine gas port puts out much higher pressure.
A large gas port on a carbine is best run with an adjustable gas block to let you throttle back the gas flow until your gun runs just right. |
Benefit #3: Healthier Dwell Time for a 16" Barrel.
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Dwell time is the delay from when the bullet passes the gas tube hole to the time the bullet exits the barrel. This stretch is actually pretty important, because the gas pressure used to cycle the action goes away almost immediately once the bullet has exited the barrel and broken the seal, so a bullet that exits too soon (too little distance between gas tube and muzzle) isn't giving the action enough time under pressure, commonly called "undergassing".
On the other hand, a bullet that takes its time about leaving (too much distance between gas tube and muzzle) is "overgassing". Overgassing sprays excess gas and debris into your action, making the gun run dirtier and giving you a headache, but it doesn't damage the weapon itself as ARs vent excess cycling pressure through holes in the bolt carrier. A 16” barrel with a carbine gas system is almost always overgassed, as it has both higher gas port pressure (from the table above) and additional dwell time (shown in the table below) compared to the original rifle-length system. |
A good way to check for an undergassed AR is to fire a shot with only 1 round in the magazine. Do this about 10 times and if the bolt doesn't lock open every single time, it's likely that the gun is undergassed for some reason and the bolt it isn't cycling all the way back (insufficient dwell time could be one of several causes).
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This overgassing in a 16” barrel carbine is often minimized by a smaller gas port, but there is little standardization in the industry regarding gas port size so there isn’t a guarantee your individual rifle will the correctly tuned. Additionally, a smaller gas port will be much pickier about ammunition choice, especially with some steel-cased ammunitions which tend to use fast-burning powders at low charges.
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Because shorter gas systems return higher pressure, the dwell time should be less than that of a full-length rifle system. You don’t need to spend as much time under pressure if you’re using more pressure than before. The mid-length gas system drops dwell time length down a full inch from the rifle-length design, which may seem excessive, but remember from the first table that a mid-length is generating about 1/3 more pressure than a rifle. A mil-spec carbine with a 14.5” barrel also returns a good result, about 0.5” short of a full-length rifle system. This is probably a bit snappier than is strictly necessary, but is still fair enough. The commercial carbine is the only arrangement that actually increases dwell time. Combined with the increased gas port pressure, this is a lose-lose scenario that offers zero advantages, and should be avoided.
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Bear in mind that undergassing and overgassing can occur in lots of ways besides improper dwell time, with causes ranging from improperly sized gas ports to poorly fitted gas system components.
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Benefit #4: Two Extra Inches of Handguard Length.
This benefit only applies if your AR’s handguards do not extend past the gas block, but that’s a big chunk of the ARs in circulation, so it’s worth mentioning. Some shooters feel cramped by the shorter carbine handguard, and certain shooting techniques ask the shooter to put their hand as far forward as possible. Adding two inches to the handguard can make the difference between “just ok” and “fits like a glove”, especially if you’re mounting gadgets like lights or lasers near the front of the handguard.
Benefit #5: Bayonets!
Will 99.9% of shooters ever have cause to use a bayonet? Definitely not. Do we still want to put knives on our guns? We sure do. Bayonets mount to the AR at two points: a lug on the gas block and a ring over the muzzle device. Put too much distance between those two points, and the bayonet doesn’t fit. The commercial carbine's barrel is 1.5" longer than the military M4 the bayonet was intended for, so the bayonet ring will rattle around on the barrel itself, but the mid-length gives enough room to mount a bayonet MOSTLY correctly with the ring on the muzzle device. So be cool, and put a knife on your gun.
Benefit #6: It just looks better...
Most shooters will agree that the aesthetics of a 16” barrel mid-length are just plain better than a 16” barrel carbine. Without that extra 1.5” on the end, it just looks more proportional. See the picture below to decide for yourself.
Now all of that aside, a 16" barrel with a carbine gas system can be a perfectly reliable system. LuckyGunner.com conducted a study using several 16" barrel carbine gas ARs with as tiny a gas port as you'll ever see, which SHOULD have made for a very touchy setup. One of these "touchy" guns fired 10,000 rounds without a single malfunction (see the article here). However, that flawless record was set using 10,000 rounds of high-quality, brass-cased ammunition. When three other identical carbine-gas ARs were set to the same task using cheaper ammunition, their performance suffered drastically, while a mid-length AR (with a wider gas port) that also participated in the testing was vastly more tolerant of the cheap ammunition.
All this goes to show that there are many factors that impact your rifle’s reliability. All the above benefits of a mid-length are nice, but for reliability, the single most important factor is ammunition quality. Load up some quality brass and you can overcome a lot of problems. However, if you plan to use cheap ammunition (and really, who doesn’t?), you should really look at mid-length or rifle length guns. LuckyGunner's study was to compare steel and brass-cased ammunition, but it also showed that the mid-length gas system maintains performance over a much wider range than the carbine gas systems. The longer, cooler-running gas systems allow larger gas ports and reap myriad other benefits, which all translate to a more reliable gun.
In summary, we can safely say that making an AR run reliably is a process with many factors to consider. To that end, the best advice in the world is simply “buy a good rifle and feed it good ammo”. Pay for quality, and you’ll likely never have a concern.
However, if you’re given the option for a 16” barrel, a mid-length gas system has several substantial advantages over a carbine length gas system. Both will work, but one will work better. Plus, mid-lengths look pretty darn spiffy…
All this goes to show that there are many factors that impact your rifle’s reliability. All the above benefits of a mid-length are nice, but for reliability, the single most important factor is ammunition quality. Load up some quality brass and you can overcome a lot of problems. However, if you plan to use cheap ammunition (and really, who doesn’t?), you should really look at mid-length or rifle length guns. LuckyGunner's study was to compare steel and brass-cased ammunition, but it also showed that the mid-length gas system maintains performance over a much wider range than the carbine gas systems. The longer, cooler-running gas systems allow larger gas ports and reap myriad other benefits, which all translate to a more reliable gun.
In summary, we can safely say that making an AR run reliably is a process with many factors to consider. To that end, the best advice in the world is simply “buy a good rifle and feed it good ammo”. Pay for quality, and you’ll likely never have a concern.
However, if you’re given the option for a 16” barrel, a mid-length gas system has several substantial advantages over a carbine length gas system. Both will work, but one will work better. Plus, mid-lengths look pretty darn spiffy…
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Background on the Mid-length Gas System (interesting but unimportant)
For starters, it is generally accepted that Eugene Stoner's original design for the AR, a 20" barrel a with rifle-length (12.5") gas system, is the most reliable arrangement. Not only is this arrangement pulling very cool, predictable gas from a large, reliable gas port, but additionally, the longer, heavier buffer of the typical rifle-length stock system adds a helpful delay to the rifle's closing stroke, ensuring that it's slow enough to be within norms. That said, carbine ARs are shorter, lighter, and arguably better as an infantry rifle, so it made a lot of sense when the military wanted a shorter M16. However, Colt engineers had a heck of a time chopping the barrel down from the original, reliable twenty inches to produce a carbine that worked consistently (the finicky AR's designer Eugene Stoner was no help, being long since out of the picture). Colt finally landed on a 14.5” barrel for the shortened M4 carbine for military use, but for civilians, federal regulations require all non-NFA rifles to have a barrel at least 16" in length, and as a result almost all civilians carbine ARs feature 16" barrels, not 14.5” barrels, and that inch and a half can be kind of a big deal.
A shorter gas system (gas port closer to the chamber) is automatically more stressed because the powder is burning hotter and harder earlier in the barrel; this is the cause of the "snappy" feel to the recoil of a carbine AR compared to the gentler "push" of a rifle AR, as the bolt's velocity is much higher and bottoms out against the rear of the buffer tube much faster and harder. Early auto-loading designs like the M1 Garand and its contemporaries usually captured gas from the muzzle for a reason: it was just too much hassle trying to pull gas from earlier in the barrel. For the same reason, some pistol-length ARs use a "pigtail" gas tube, adding extra length to the gas tube to allow the propellant gases additional time to cool before reaching the piston chamber in the bolt. As a result of its shortened gas system, an M4 carbine operates at almost double the pressure of an M16 rifle, so Colt engineers had to not only master the dwell time, but find other ways to arrest bolt travel and reduce internal stress caused by the carbine's shorter gas system. Adding to this problem is that the pressure generated by low-quality ammunition is especially prone to spike and drop erratically in the first few inches of barrel, as shown in the Lucky Gunner article, although this is only a problem for civilian shooters (the military hands out pretty good brass).
Against it all, the Colt boys were able to smooth things out and get the now-standard 14.5" barrel M4 dancing a merry jig using a gas port at the 7.5" mark (to get the dwell time within limits), M4 feed ramps (to accommodate the increased cyclic rate), heavy-weight buffers (to compensate for the increased pressure), and all the other assorted nubbins and tweaks that are now commonly known to make carbine ARs more reliable. Yet, a somewhat substantial monkey wrench is thrown into the equation when you add 1.5" to the barrel to make it a civilian sixteen incher. It's not much, but it's enough, and that's where the mid-length gas system comes in. The middy gas system moves the gas block far enough forward to get the dwell time within limits on a 16" barrel, and simultaneously allows a larger gas port to tap cooler gases, and thus is a development popular with the civilian AR. Although you don't need one, it will definitely help, particularly if your AR isn't a top-shelf design and/or you are shooting cheap ammunition. As a final bonus, middy's just plain look better without all that extra barrel sticking out front.
Now comes the classic rebuttal: "Well I have a 16" barrel with a carbine gas system, and it runs like clockwork." Indeed you may. The flawless record of the brass-cased ammo AR in the LuckyGunner article shows that 16" carbine gas can be a very reliable setup, because the gas system is only one of many elements contributing to a reliable weapon. Imagine the 16" barrel AR as a fast car, and the mid-length gas system as good tires. Good tires are important, but a car can still be pretty fast without them, as long as it has a good engine, a stiff frame, and a skilled driver, because the tires are only one element of a truly fast car. However, if you want that "fast car" to run at its true potential, you better get yourself some good tires.
So there you have it. Mid-length gas systems, an elegant solution to a problem created by legislation, not battlefield doctrine. When you've got a 16" barrel, you might not need it, but it'll help to have it, and they look pretty spiffy to boot.
A shorter gas system (gas port closer to the chamber) is automatically more stressed because the powder is burning hotter and harder earlier in the barrel; this is the cause of the "snappy" feel to the recoil of a carbine AR compared to the gentler "push" of a rifle AR, as the bolt's velocity is much higher and bottoms out against the rear of the buffer tube much faster and harder. Early auto-loading designs like the M1 Garand and its contemporaries usually captured gas from the muzzle for a reason: it was just too much hassle trying to pull gas from earlier in the barrel. For the same reason, some pistol-length ARs use a "pigtail" gas tube, adding extra length to the gas tube to allow the propellant gases additional time to cool before reaching the piston chamber in the bolt. As a result of its shortened gas system, an M4 carbine operates at almost double the pressure of an M16 rifle, so Colt engineers had to not only master the dwell time, but find other ways to arrest bolt travel and reduce internal stress caused by the carbine's shorter gas system. Adding to this problem is that the pressure generated by low-quality ammunition is especially prone to spike and drop erratically in the first few inches of barrel, as shown in the Lucky Gunner article, although this is only a problem for civilian shooters (the military hands out pretty good brass).
Against it all, the Colt boys were able to smooth things out and get the now-standard 14.5" barrel M4 dancing a merry jig using a gas port at the 7.5" mark (to get the dwell time within limits), M4 feed ramps (to accommodate the increased cyclic rate), heavy-weight buffers (to compensate for the increased pressure), and all the other assorted nubbins and tweaks that are now commonly known to make carbine ARs more reliable. Yet, a somewhat substantial monkey wrench is thrown into the equation when you add 1.5" to the barrel to make it a civilian sixteen incher. It's not much, but it's enough, and that's where the mid-length gas system comes in. The middy gas system moves the gas block far enough forward to get the dwell time within limits on a 16" barrel, and simultaneously allows a larger gas port to tap cooler gases, and thus is a development popular with the civilian AR. Although you don't need one, it will definitely help, particularly if your AR isn't a top-shelf design and/or you are shooting cheap ammunition. As a final bonus, middy's just plain look better without all that extra barrel sticking out front.
Now comes the classic rebuttal: "Well I have a 16" barrel with a carbine gas system, and it runs like clockwork." Indeed you may. The flawless record of the brass-cased ammo AR in the LuckyGunner article shows that 16" carbine gas can be a very reliable setup, because the gas system is only one of many elements contributing to a reliable weapon. Imagine the 16" barrel AR as a fast car, and the mid-length gas system as good tires. Good tires are important, but a car can still be pretty fast without them, as long as it has a good engine, a stiff frame, and a skilled driver, because the tires are only one element of a truly fast car. However, if you want that "fast car" to run at its true potential, you better get yourself some good tires.
So there you have it. Mid-length gas systems, an elegant solution to a problem created by legislation, not battlefield doctrine. When you've got a 16" barrel, you might not need it, but it'll help to have it, and they look pretty spiffy to boot.
Author's Note
If you don't want or can't find a mid-length, a solid alternative is a carbine gas system with a 14.5" barrel and a permanently pinned, >1.5" long muzzle device. This arrangement gets rid of the commercial carbine's excessive dwell time. Outside of suppressors, most muzzle devices won't bother the gas system much and as long as they're pinned and welded in place, the barrel will meet the legal minimum of 16". However, if you want to swap your muzzle device from time to time, or install a free float rail, or really mess with just about anything on the front end, you'll probably need to remove the muzzle device. Unfortunately, getting a pinned muzzle device off and back on again without damaging the barrel is a risky and often expensive operation, and the installed muzzle device is usually ruined in the process.
If you don't want or can't find a mid-length, a solid alternative is a carbine gas system with a 14.5" barrel and a permanently pinned, >1.5" long muzzle device. This arrangement gets rid of the commercial carbine's excessive dwell time. Outside of suppressors, most muzzle devices won't bother the gas system much and as long as they're pinned and welded in place, the barrel will meet the legal minimum of 16". However, if you want to swap your muzzle device from time to time, or install a free float rail, or really mess with just about anything on the front end, you'll probably need to remove the muzzle device. Unfortunately, getting a pinned muzzle device off and back on again without damaging the barrel is a risky and often expensive operation, and the installed muzzle device is usually ruined in the process.
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