As a quick refresher, the gas port is a hole drilled into the barrel that allows some of the burned gunpowder’s gas to cycle back into the gun’s action. The gas is funneled into the bolt carrier, and the pressure of the gas cycles the action, ejecting the empty case, resetting the hammer, and compressing the action spring. The action spring then decompresses, pushing the bolt carrier forward, stripping a new round out of the magazine and loading that round into the chamber, making the gun ready to fire once again.
*cycling action diagram stolen from: https://boltcarriergroupblog.wordpress.com/so-how-does-the-bolt-carrier-group-work-within-the-ar-15-system/
Now that we’re all on the same page, let’s get to it.
‘Dwell time’ is a term that most gun people never come across, though it’s not a difficult concept. Dwell time is the amount of time a bullet spends between the gas port and the muzzle. It is, obviously, an extremely short yet important amount of time. Dwell time is important because the pressure of the gas stuck behind the bullet is what cycles most semi-automatic and fully automatic guns, though it isn’t really necessary to understand outside of very specific circumstances. The modern AR-15 is one of those circumstances.
Dwell time was a concept that really only mattered to people designing guns, until recently. Most guns people bought were complete from a manufacturer. Sure, you could get trigger jobs and accessories and optics and stocks and personalize the weapon to your liking, but for the most part the internals and parts necessary to the actual function of a gun were pretty much set from the factory. In fact, most guns that aren’t ARs are still this way. Since those guns are made in one place by one manufacturer, they (if they are even halfway reputable) have to make sure that their guns function reliably. If the gun is semi-automatic, that means that the factory has to make sure that the gun has enough burned gunpowder gas to work the action, but not so much that the gun beats itself to death (known as “overgassing”).
Basically if you buy your gun as a whole from a factory, dwell time has been figured out and you don’t need to worry about it. Ditto for complete AR uppers (exception below).
The only time a person really needs to worry about dwell time is if they are changing major operational aspects of a gun’s gas system, or designing a weapon from the ground up. If you are parting together an AR upper, you fall into one of those groups.
In ARs, the dwell time really only matters from when the bullet passes the gas port to until it exits the muzzle. The dwell time is determined by the bullet’s speed, acceleration, and the distance between the gas port & muzzle. Since 2 of those 3 things are determined exclusively by the cartridge being loaded into the gun, this post will focus on the distance between the gas port and the end of the barrel.
The different gas length systems were developed and are intended for different length barrels, and is illustrated in the image below. The rifle length gas system was designed for use in 20″ barrels (top). The carbine length system was designed for the military’s 14.5″ M4 Carbines (bottom), and the midlength system was designed for commercial civilian guns running 16″ barrels (middle). There is also a pistol length gas system, and it was designed for use on very short barrel lengths, like barrels 9″ and under.
For the purposes of this post ,”gas system” will refer to the distance from the chamber to the gas port (aka ‘port length’), and the length of the gas tube running from the gas port back to the upper receiver. Helpfully, these two lengths kind of have to be the same.
The dwell time can be visualized as the time it takes the bullet to travel between the gas block and the end of the barrel. When shooting the same ammunition side-by-side, the carbine system would have much more gas flow back into the upper receiver than the mid-length system, and the midlength more gas than the rifle system. This may not be immediately apparent, so let me explain.
In the image above, all three barrels have roughly the same distance between the gas block and the muzzle. If the dwell time is determined by this distance, and the distances are similar, why aren’t the dwell times and the gas volume delivered to the BCG the same?
The pressure of the gas is not uniform throughout the barrel. When the primer is struck and starts to ignite the gun powder, the pressure soon spikes and maxes out. The first inch or two of barrel experience the highest pressure in the entire firing cycle. As the bullet moves down the barrel, it is giving the burning gasses behind it more room to expand into, lowering the pressure of the gas behind it. Since the pressure is still measured in the thousands of pounds per square inch, the bullet will continue to accelerate. As the bullet continues to go faster and faster, the room that the gasses can expand into increases at a faster and faster rate, thereby dropping the pressure exponentially as the bullet travels, until it finally leaves the muzzle and dropping the pressure effectively to zero.
So, not only does a shorter gas system allow gas to enter the action sooner, and allow more time for gas to enter the action, but a shorter gas system allows more gas to rush in under higher pressure, compounding the effect.
Remember, not only does the gas enter the gas system later in a longer system, and at a lower pressure, but that gas has a longer distance to travel back through the gas tube to get back to the receiver, further lowering the pressure available to act upon the bolt carrier.
That doesn’t mean that people always use the correct gas system for their barrel, though. Most 16″ barrels on the market have carbine length systems on them. There’s nothing wrong with that, but that gun will be overgassed with anything but the lightest, softest loads you can buy. There’s even a case where a well-known firearms trainer spec’ed out a “marksman rifle” with a 20″ barrel and a carbine length gas system. You want to talk about overgassed? Because this is it:
I know what you’re thinking: Why does this matter? Why should I care about matching my gas system to my barrel length? Because of longevity of the rifle.
Over gassing a rifle will always lead to a shortened life span. It can mean you break parts in the rifle sooner, parts wear out and need replacement quicker, and more felt recoil. Overgassing a rifle makes it work harder than it needs to. [UPDATE 6-4-2018: The Navy is currently running tests on carbine and mid-length gas systems, and looks to be agreeing with me.]
On the other hand, undergassing is when the rifle doesn’t get enough gas to function properly. An undergassed gun will malfunction much more often than it should. It might have failures to pull the empty case out of the chamber, fail to eject the empty completely out of the action, might not even be able to strip the next round out of the magazine.
Why do manufacturers pump out 16″ barrels w/ carbine gas systems 10-to-1* compared to every other configuration out there? Because of cheap ammo. Most people who own ARs buy the cheapest ammo Wal*Mart has on the shelf, Tula and Wolf. These are steel cased rounds out of Russia and Eastern Europe that are absolutely dirt cheap and are loaded very light, so light that they can make a gun that has a proper barrel & gas system undergassed. Most manufacturers have determined that instead of fielding tons of customer service calls and returns of properly tuned guns that choke on cheap ammo, they’ll just overgas the guns from the factory so they’ll run no matter what you put in them.
*totally made up number
Most manufacturers have determined that they’d rather have their guns fail a couple thousand rounds sooner due to the overgassing than have their guns fail out of the box due to undergassing. I really don’t blame them. The situation further supports their decision by the fact that most people don’t shoot their guns’ entire lifespan. A mil-spec AR barrel has a lifespan of about 20,000 rounds. Most ARs in this country will see under 1,000 in total. So what if the gun you produce will break a bolt carrier or shear a bolt lug every 4,000 rounds? Only 1% or less of your product is reaching that round count. You could replace all those broken parts for free and still be money ahead, compared to the customer service calls and returned guns that function with decent ammo.
It’s not ideal, but it’s the world we live in.
In conclusion, configure your rifle with components that are meant to be put together. First, determine your barrel length, then determine your gas system, and then find someone who is selling that barrel + gas system.
For 18″ & up: rifle length
for 14.5″ to 18″: midlength
for 10″ to 14.5″: carbine
for under 10″: reconsider your options.