David, no argument from me. I only point out that Brophy suggests that Wild Fowl guns are chambered for 3.0 inch shells. Obviously not all of them are. My Wild Fowl gun does not have the 3 inch chamber stamp or the reinforced splinter. Brophy also provides drawings and charts that give the minimum chamber diameter of Smiths as .797(1907), .795(1936), and .792(1938). Because of the angle of the forcing cone, the chamber measurement will not be greatly affected by the small variations in gauge diameter. I elected to use the SAMMI diameter as my standard. You are free to use any of the Smith standards. In the end, if we want to compare data, a single standard simplifies data comparison. A suggestion on making gauges. I made a long gauge (3.5 inches) and then use a depth micrometer to measure the distance from the end of the gauge to the end of the barrel. When the depth gauge reading is subtracted from the length of the gauge, you will have the chamber length. With this method, one gauge can be used for any 12 gauge barrel.