There are a variety of gas system lengths that can be used on custom barrels.
The picture above illustrates all of them and it is drawn to scale.
Various cartridges work better with specific combinations of barrel length and gas system.
There are too many combinations to list them all here.

Keep the following in mind when you are deciding what length gas system to order:
Proper function depends on the right amount of gas getting into the action.
We can control gas flow by the port diameter and by the amount of 'dwell time'.
Dwell time is the duration where the bullet is in the bore after it passes the gas port.
The gas starts to flow as the bullet passes the gas port and stops when the bullet exits the muzzle.
The original 20" rifle design has 6.8" of barrel out in front of the gas port and the M4 has 6.7"
As a general rule, you get best function with at least 5" of barrel in front of the gas port.
Gas port size is also used to adjust function when a non-typical gas system length is used.
As a general rule, barrels with smaller diameter ports and more barrel out in front work with
a wider range of ammo than barrels with a larger port that's closer to the muzzle.
You can go as short as 3" and still get function, but the range of functional loads will usually be smaller.

See the Quickload plot below for more info on how gas port pressure and location relate to functioning.