I asked for the chamber front end diameter. Our late friend Oscar gave me .798"(12 ga) that has worked for me. Chamber length is measured from the breech end of the barrels to the beginning of the forcing cone.
I don't really understand your last paragraph. Chokes are indeed measured in thousandths of an inch. The bore I.D. must be known. Subtracting the choke I. D. from the bore I.D. gives you the choke constriction. Verify it on paper patterns if you like but the accepted measurements will satisfy most people.
