ORIGINAL: outdoorsmen
the ribs are for filling the gap between the slug and the barrel due to various bore diameters among manufactures as well as choke sizes. the ribs do not invoke spin on the projectile.
Then why do they call them "rifled slugs" and why are the ribs slanted? Las time I cut apart a slug to look at it looked like the opposite match of a .45acp barrel? The lands and grooves of the pistol barrel invoke spin so why not the same on the slug?... Also the difference in bore diameter and choke consrtictions from different manufactures is negligable unless specified as "overbored"...