Khan;
20.3 mm would convert to .799" which is about spot on for the small/front end of a 12 gauge chamber (Current SAAMI specs list .798" minimum). When I stated this was the dimension at the Rear of the chamber that was actually a Belgian proof house method for some years, apparently the Italians mark the forward end. Would have to dig out some of my old proof info to find for sure.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra