Don't guess or estimate the internal bore diameter when you can know the exact measurement down to one-thousandth of an inch. Inexpensive, accurate, precision micrometer makes accurate measurements of bore, choke constriction, or forcing cone length....
One gauge set measures the inside bore diameter of all shotgun gauges, from the little 28 to the big 10 gauge (except .410 bore). Accurately reads bore and choke constriction, chamber diameter or forcing cone length. Even helps identify bent or damaged...