Here is the procedure I use ( I don't have a dual regulator but I do have a manifold that splits the CO2 going to the keg and a separate connection that I use to connect to the BeerGun):
Get the bottles cold,
Turn off the gas to the keg,
Bleed head pressure from keg,
Set pressure regulator to 2-3 lbs (just high enough to push beer from keg to bottle),
Purge bottle with CO2,
Fill bottle,
Cap on foam.
Using this process, I've opened bottles a year after filling and the carbonation was still good.