This is the first of an N part series aimed at deeper understanding of how our rifles work.
There is no problem to be solved.
My personal ARs run just fine.
All of the following is believed to be true but subject to review.
Comments and discussion are encouraged.
Let’s talk about gas ports.
Gas ports used in various systems range in diameter from around .043" to .100".
This range of diameters may not sound like much difference, but their corresponding areas have a difference of over 4 times.
This is important because the gas port area controls the mass flow rate of gas to the piston system.
Taking some practical numbers, if a .0625" port is taken as a baseline, an .089 port has twice the area, and an .043 port has half the area.
Let that sink in.
In a general sense, every system uses three gas ports and a piston chamber.
There is the fill port, generally made directly into the barrel.
There a vent or bleed port, which can take many forms, such as actual holes or leaks and loose fitting parts.
Lastly there are the dump ports that are exposed after the action has accelerated to full velocity and moved some short distance.
The fill and bleed ports work together to produce the actual pressure that is applied to the piston.
This is the basis of most adjustable gas systems with a twistable adjustment.
By varying either the fill size or the vent size, or both, different pressures can be produced in the gas system.
In the case of steady state flow with no moving parts, which never happens during cycling, the pressure in the piston chamber is found by taking the applied bore pressure times the fill port area divided by the total port area.
Stated simply, if the fill and vent ports are equal size, the piston chamber pressure will be half ( 1/(1+1)).
If the fill port is small (.043") and the vent port is normal (.063), you get 33% pressure.
You can get the same result with a normal (.063) fill port and a large (.089) vent port.
This 33% pressure is actually 66% of normal pressure, and is handy to counteract suppressors, which extend the time that the gas system is at useful pressure by up to 1.5 x