Huntschool- It is flat bottom rifling and about 1 in 66" twist. To measure it, I have a .30 caliber military surplus steel cleaning rod with a swivel handle. I pinned a square nut next to the handle so I can turn the rod to tighten the cleaning jag. Pushed the jag clear to the breech with a tight patch. Marked the rod at the muzzle. Turned the rod to make the top of the nut level. Pulled the rod out 90 degrees so the next side of the nut was level. Measured the pull out, which was 17 inches. Seventeen times four (for a full turn) is 68 inches, but this measurement is approximate, so I guess the twist is probably one in 66, a common one.