I'm thinking that he doesn't have a choke tube but is asking what choke his 20 ga is based on the measurement he took. Is that a open, improved, mod, or full choke diameter? You math guys should be able to convert the measurement to choke size based upon known barrel data. I checked my 20 ga mod barrel at .610. Could be a loose mod or improved cyl choke. That's as close as I can get on this one.