I took delivery of the 16-12 gauge bore skeets bore gauge and was disappointed that it wouldn't fit into the left barrel of a 16 ga. Robust. The diameter of the brass structure which contains the spring loaded ball bearings is .623 and according to my calipers, the full choked muzzle diameter of the left barrel is .615. Bear this in mind if you intend to measure European barrels with the 16/12 Skeets instrument. Gil