My 16 Ideal Grade Ser # 2029XX was measured two weeks ago by David Trevallion with his precision gauges and found to be a little over 2 1/2". I stood over his shoulder and wanted to say "do it again and again if necessary." I tried the index card way and found I could get a variety of measurements. Reading Shotgun Technicana and getting a little understanding about chambers and forcing cones it doesn't seem as though you can measure with an index card and distinguish a difference of 1/4" because of the tapers involved. I would have been happier if he told me 2 3/4" as the gun was built very late 1943. I was under the impression that all guns after about 1939 were 2 3/4". But I also have a Field Grade 2028XX that doesn't have the single sighting plane rib. Go figure.