I have a question related to the AR20 regulator (might apply to the AR10, as well):
If you remove the cap (which is also the pressure regulator) to reveal the hex screw, you'll notice a tiny hole near the center, that goes down into the regulator itself.
Days ago, while testing for leaks in my tapped magazine (long Tanaka mag) I accidentally submerged the AR20 regulator as well and noticed that some tiny amount of air bubbles were coming out of that tiny hole.
My guess is that, since the AR20 caps at around 120-140PSI (mine does, at least), and since my primary regulator is pushing air at 200PSI, the excess pressure is so high that - in order to regulate it - the AR20 uses that tiny hole as an exhaust valve. This also ensures the regulator does not blow up when feeded with very high pressures.
Now, the above is my personal guess
..although I do have some common sense, I might very well be wrong so I am asking you guys to back me up on this or bash me down
Thanks for the help!