I have posted the use of a 6" machinist scale here several times. It is very unlikely you will be able to "Feel" the beginning of the cone. Instead with bbls removed point them toward a light source, as a window. You can get your eye behind the bbls & looking inside see the cone as a shadow. Slide the scale in so you can still see the cone & you can watch for the corner of scale to just touch the shadow line. Do this several times till you are sure you are getting consistent readings & you will quickly see you are well within ±1/32" which is all the accuracy you need for this job. A precision made chamber gauge which just happens to measure .001" larger than the chamber end (Not at all an unusual condition on old chambers) will show far more error than this.
