I assume that a nominal 2 3/4" chamber would actually measure 2 5/8" as was common at the time. A tight 2 5/8" chamber may measure less than that with some chamber gauges. The only way to tell when the chamber actually starts is by watching a bore micrometer as it enters the forcing cone. Unfortunately, most 12 gauge bore micrometers will not extend to the diameter of a 12 gauge chamber. The alternative is to view a machinist's scale as it enters the forcing cone, a difficult process, but possible. I would like to hear Frank Srebro's take on this, since he has measured a bunch of HE Fox chambers.
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