I like to use Hosford Barrel wall thickness gauge for checking chamber length. Watch the needle closely it will change directions at the end of the chamber then read the scale on the rod and you have the chamber length. This method gives an accurate reading even with a chamber that is at the minimum or maximum of the chamber diameter spec. Its also good to know the thickness at the end of the chamber if it has been lengthened .
I use the Galazan pocket gauges and the black plugs for quick checks but they can lead you astray with a tight chamber.
