12 gauge 3" chamber dimensions are not listed in the L C Smith Plans & Specifications book. Drawings for the 2 3/4" show a chamber dimension at the cone of .797" diameter 2.750" from the breech. This drawing is dated Dec 21 1907, definitely Fulton Era. Taper in the chamber body is .014" over the 2 3/4" length. I believe that present day SAAMI specs call for a minimum diameter at the cone of .798". Note that if the chamber gauge is made to the minimum dia of the chamber this .001" smaller chamber will cause it to stop short by .196" or a bit over 3/16". When I made my chamber gauges I made them to these minimum chamber diameters from SAAMI. I very quickly found them to be totally worthless on about 75% of all the older doubles I tried to check with them.
The "Plans" book in a drawing dated 6-27-40 for a 12 gauge chambering reamer calls for the reamer diameter at this point to be .795", thus it would have to cut .002" oversize to cut a .797" chamber, not really likely. Flutes are 5˝" long so it could cut any length chamber desired & could also be resharpened many time without losing size as it would be simply stepped back & shortened a bit.
