I measured mine with a calibrated mitotoyo inside micrometer and it is .685 at the breech (where the ball enters) and .6865 at the muzzle (where the ball leaves) before the muzzle break
One other thing, Ive shot a half case of Kick'N Mettalix (Medium Bore) thru mine and the paint was SUPER tight. I thought it would break, in fact I was sure it would but it didn't and it was very nice. The only thing I noticed was on some of the balls, near the end of travel, would slightly spin off in any direction. If you have a good paint/barrel match it should work great.