The Fallacy of the Ideal Gas Port
With the advancement and mainstreaming of the short-barreled AR-style carbine and the proliferation of silencers, much work has been done to overcome the negative effects that come with short barrels and suppressors. With short barrels come short gas systems. With shortened gas systems come higher port pressures and diminished dwell time. With suppressors come an increase in the duration of system pressurization, both through the gas tube and through the bore (manifested as what's often called “back pressure”) as the escape of pressurized gasses is purposefully slowed to reduce the sound signature of each shot. All of this results in accelerated unlocking, violent extraction, increased recoil, and a number of other negative effects.
In an attempt to mitigate these detrimental factors, industry has gone to significant effort to refine, tune, or optimize gas port sizes for acceptable performance under common or selected conditions and configurations. You'll see this in the form of barrels, gas ports, or gas blocks advertised as having "reduced" or otherwise tuned port sizes. The idea is that the gas port can be sized to deliver reliability under one set of nominal conditions and probably work under most other common conditions, assuming the use of ammunition that delivers a consistent level of pressure, that a suppressor is consistently in use or not in use at all, and that environmental conditions reach neither extreme of hot or cold. This method, combined with the selection of heavier or lighter buffers and varied spring rates may indeed result in acceptable reliability in most conditions.
What simply can’t happen with a single gas port size, however, is the achievement of ideal performance under all conditions, with all ammunition, with or without a suppressor, at the least amount of recoil and aiming disruption. There is no such thing as one ideal gas port size. As we’ve detailed in other parts of this blog, the AR requires a balanced system to perform with maximum reliability and minimum recoil for the longest possible service life. When one leg of the triangle of factors that determine AR performance (Pressure, Time, and Mass) is increased or decreased, one or both of the other two must also change for ideal performance to be reestablished (if it can be established or reestablished at all with the components in use). Any one gas port size, regardless of how ideal it is with a given type of ammo, suppressor, or set of conditions, will always be and can only be a compromise. Change ammo? Be prepared to change your buffer and/or buffer spring. Set up your carbine during the hot summer months? When winter comes you should be prepared to change your configuration again.