As indicated above, the drop-in gauge only works if bore diameters are "standard" (12ga = .729, etc). I have a German 20ga that's a solid improved cylinder (.007) in one barrel. My drop-in Galazan gauge shows no choke. Because that particular gun is marked 19 on the barrel flats, I know it's overbored by at least .010 from the 20ga standard of .615--so the overbore results in an incorrect reading. You need a tool that measures both bore diameter and constriction.