Hal;
I habitually measure shotgun chamber lengths in essentially the same method you describe. I use a 6" flexible steel scale & slide it along the chamber wall until its corner touches the shadow of the cone as you described, then just read the measurement on the scale. I have "Definitely & Absolutely" in spite of what some may tell you, found instances where this was more accurate on older guns than using a chamber gauge. This was due to the chamber diameter being slightly smaller than the "Modern" gage thus causing the gage to stop short in the tapered chamber.
Understand the shell length is determined by its "Fired" length. Due to the amount of case used in making a fold crimp even a 3" magnum shell would ordinarily enter a 65 mm (Aprox 2 9/16") chamber with no resistance, But should not be Fired in same.
