Anybody want to explain to me what and how "Air Gauging" a barrel works and how accurate is it ?I've seen this forever with Douglas barrels but haven't a clue

Rick

For those that don't want to watch youtube, air gauging works by putting a button slightly smaller than barrel diameter into the barrel, and introducing presurized air  behind it, while monitoring the pressure.  The button is moved down the barrel, and the air pressure changes are graphed vs button location in the barrel.  As the barrel diameter changes, the leakage around the button causes pressure changes.   

The system has been calibrated for button size vs bore size vs pressure to indicate variatons in diameter.   

Barrels with the least variations in pressure and thus diameter are more consistent performers.  Douglas has established a pass/fail criteria for it's air gauge barrels to set the level of quality.

I always wondered about that.   

I have a Green Mountain barrel, which I heard are air gauged as part of the process, but it has a rifling deformity that is huge.  When you push a tight patch down it, it feels like a big bump when you hit the really bad patch.  It is also full of loose spots which are easy to feel when slugging it.  Shouldn't air gauging have caught that?

Or is air gauging done through the bore before rifling it?

Chris.