Works in theory but doesn't account for eccentricity of bore to exterior caused by "striking" (filing to finish O.D.). As a matter of verifiable fact, no W/T gauge has access to the area joined by the ribs. If frugality is the aim, Rookhawk, you could money back that brass "ring sizer" to Tony G and measure the differential bore to choke with your Skeets bore mic. If you want to build a better mousetrap like that of James I. ,a simple needle indicator from Harbour Freight is even cheaper than a digi--probably about 15$. Pattern board will sure show you the spread and the usable concentration. Enjoy your new toys!

jack