The writeup I went through (Husky's site) noted the M17's with rifled 20 and 13 bores(mine) were meant for BP and round balls in brass cases, with paper cases for shot loads seems to me.
The only I had, the right barrel was rifled (not the left) and while it handled a 16 bore brass case, a groove diameter ball was .705".
The right barrel was larger in diameter than the left 16 bore mod. choked barrel all the way from just ahead of the breech, to the muzzle. The rifling was straight in this gun.