Hello David, and thanks. I don't have the ability to measure actual bore diameter, but I can accurately measure constriction as the difference between the diameter at muzzle and diameter at cylinder bore with a Skeets bore gauge. As I indicated, the constriction between dead end at muzzle and where it becomes a constant diameter at 7 3/4" back from muzzle is 59 thousandths. The diameter remains constant from the 7 3/4" point to the end of the forcing cones. The resulting absolute value of the choke, or total constriction between cylinder bore and muzzle end is 59 thousandths, unless I have a misunderstanding regarding what actually constitutes choke. (Haha, and that wouldn't surprise me at all!) Thanks again, John