Gas System Length
Facts and Fiction
A 16 inch barrel with a Carbine length gas system shown above a 20 inch barrel with rifle length gas system to show remaining length beyond the port.
There has been a lot of info written recently about gas system lengths, and some of it is legitimate. However, a lot of what's written about WHY they work differently is not based on scientific principles. For instance, a lot of people think that moving the gas port further down the barrel is done to reduce the amount of pressure in the gas system. This is not true! While it is true that the pressure in the barrel drops as you move further away from the chamber, the pressure applied to the gas system is controlled entirely by the gas port diameter. The cross sectional area of the port determines how much gas gets through. Pressure is measured in pounds per square inch (PSI), and if you increase the square inches of the port, the amount of gas in the system increases. This is why rifle length barrels ALWAYS have a larger port than carbine length barrels. Since the PSI in the barrel is lower, you need a larger port to get enough gas into the operating system. A typical rifle port is about .096, while carbines are usually about .062 (each manufacturer has their own spec).
The reason the mid-length gas system was developed is dwell time. This is the amount of time the bullet remains in the barrel after the gas port is pressurized. On the original rifle length (20 inch) design, the distance the bullet travels after passing the gas port is only about 7 inches. On the later- developed M-4 carbine (14.5 inch barrel), it is ALSO about 7 inches. The problems started when civilian carbines became popular. Limited by federal law to a minimum 16 inch barrel length, manufacturers retained the standard carbine length gas system for commonality of parts, but increased the barrel length nearly 2 inches. The result is almost 9 inches of bullet travel with gas pressure in the system. By applying the same amount of pressure for a MUCH longer duration, stress on moving parts is greatly increased. This is compounded by the fact that since the port is much closer to the chamber than on a rifle, residual pressure is still very high when the gas system starts trying to open the bolt. The result can be as severe as bolt lug shear, though typically failures are less disastrous. The Mid- length gas system relocates the port about 2 inches further down the barrel, which (surprise, surprise!) leaves about the same amount of dwell time we started with.
It has been discovered that, to a point, dwell time can be DEcreased without any loss of reliability. The mid-length system works just fine on a 14.5 inch barrel, and some companies use it on even shorter barrels. A side benefit of this seems to be a slight increase in accuracy potential. The theory goes that the vibrations caused by the pressurization of the gas system have less time to affect the bullet before it leaves the barrel. In any event, many high-power rifle shooters have been increasing the gas system beyond even rifle- length in classes where it is legal.
So to sum it all up, carbine length systems are not well suited for barrels longer than 14.5 inches, Mid-length systems work well from about 14.5 up to about 18, and any barrel 18 inches or longer is a good candidate for rifle length gas system. Long competition barrels can go even longer.