Drew's method with the index card sounds interesting, will have to try it sometime. My method which I have used for some time is to use a 6" flexible scale. Doesn't really matter if it is a high quality machinist's scale or one you pick up at a home improvement center. Hold the barrel up & look into it till you see the cone as a shadow. Slide the scale in beside your eye keeping its edge against the chamber wall until the corner meets the shadow. Read the scale at the breech at that point. I have actually on several occasions found this method to be more accurate than the gauges due to variations in chamber dia.
