>>64012466
SCAR gas port on the actual barrel is 3mm
Gas jets thread into the gas block and allow a % of that 3mm through. Generally between 1.5 to 2mm worth of that gas passes into the two position regulator which can then be used to further restrict it.
Shorter barrels get a larger gas jet to compensate for less dwell time.

https://youtu.be/sInna16IFHY?t=352
Gas jet sits between the gas port and the gas regulator.