ar-15, gas-systemWhat is the difference between a carbine and mid-length gas system on an AR-15? Are these just marketing terms?
The difference is the length of the gas system. In the case of a mid-length, the gas system is 9” long. Mid-length gas systems provide lower pressures and more “gentle” cycling compared to carbine gas systems. Mid-length gas systems are advantageous for barrel lengths between 14.5” and 18” (inclusive).
The 14.5” barrel length benefits from a smoother recoil impulse and lower pressures thanks to the greater allowed expansion of the gasses in the barrel and decreased dwell time1. The 16” barrel length benefits in the same way, as well as — in my opinion — looking much better as there is less naked barrel sticking out beyond the gas block. 18” barrel lengths benefit from the opposite: an increased dwell time compared to the rifle-length gas system, and this helps improve reliability.
Other lengths of gas system include pistol-length and rifle-length. A carbine-length gas system is 7” long, meaning that the distance from the gas block on the barrel to the upper receiver is 7”. Mid-length gas systems are 9” long, rifle-length systems are 12” long.
The US Military uses carbine- and rifle- length systems on the M4 (14.5” barrel) and M16 (20” barrel), respectively. Almost all ARs built use one of the three (carbine, mid, and rifle) popular lengths. Pistol is used for extremely short barreled rifles, or “pistol” ARs (technically, it’s possible for an AR-15 to be a pistol if you classify it as such when you manufacture it, and do not ever attach a buttstock, but that’s a whole other subject).
Gas system refers to the entire system that operates the rifle, starting at the gas port in the barrel, where gas that propels the round out of the chamber is siphoned off through the gas block (which is usually also the front sight in a standard AR configuration) and sent back toward the bolt carrier inside the upper receiver. The gas is fed into what’s called the gas key on the bolt carrier (the thing on the top toward the front, an elevated tube with the opening toward the bolt) which allows the next steps in the cycle of operation (unlocking, extracting, ejecting, cocking) to take place.
1: Dwell time is the amount of time the bullet stays in the barrel after it passes the gas port and begins sending gas back to operation the weapon.
Bryson already gave an excellent answer, but perhaps I can add a bit of history to explain how we ended up with three different length gas systems.
As Bryson explained, dwell time is the amount of time the bullet stays in the barrel after it passes the gas port. Ignoring some variations in bullet velocity with barrel length, we can think about dwell time in terms of how long the barrel forward of the gas port is. The other major variable here is how much pressure there is at the gas port to apply force to the bolt. The closer you are to the chamber, the greater the pressure, the further you are, the lower the pressure.
Eugene Stone designed the original M16 with a 20” barrel and a gas port about 13” from the chamber, giving 7” of dwell time. This is probably the best setup from a pure reliability standpoint, but it’s a fairly long weapon, not the best for fighting inside buildings and the like.
During the Vietnam war, Colt went through several less than satisfactory designs for shorter versions of the M16 before settling on the Colt Commando (also known as the XM177 or the GAU-5/A). It had a 11.5” barrel with a gas port 7.5” from the chamber. Note that compared to the rifle this setup has the gas port closer to the chamber (higher pressure) but this is offset by a shorter dwell time (that pressure is acting for a shorter period of time).
When Colt developed the M4 Carbine, they took the Commando and lengthened the barrel to 14.5 inches, while keeping the gas port 7.5 inches from the chamber. This makes it possible to mount a bayonet on the carbine because the end of the barrel is the same distance from the gas block as it is on the rifle. Comparing this to the M16, we have a gas port closer to the chamber (higher pressure) but it now has the same 7 inches of dwell time as the rifle. This makes the M4 somewhat overgassed compared to the rifle, but it still works fairly well.
The military’s widespread adoption of the M4 created civilian demand for a look-alike. However, the National Firearms Act says that a rifle with a barrel shorter than 16” is a Short Barreled Rifle, requiring a bunch of paperwork and paying a $200 tax. Thus most manufacturers of M4geries added another 1.5 inches to the barrel to meet the 16” minimum. Now we’ve got a 16” barrel with a 7.5 inch gas system. The gas port is closer to the chamber than on an M16 (higher pressure) and the dwell time is longer. This creates a substantially overgassed rifle.
This finally brings us to the midlength gas system. Manufacturers recognized the issues with 16” barrels and carbine length gas systems. In response, they moved the gas port forward two inches (to 9.5” from the chamber). On a gun with a 16” barrel, this produces a rifle with a gas port that’s closer to the chamber than an M16 (but further than a carbine), and has a dwell time about on par with the rifle.
The carbine length gas system on a 16” barrel is essentially an accident of history. Now, this isn’t to say that a 16” with a carbine length gas system won’t run, but it will shoot a bit harder and suffer a bit more wear and tear than a similar gun with the midlength gas system.
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