If you fire shot loads through a rifled shotgun barrel, the shot piston is gripped by the rifling, and spun. Upon leaving the barrel, the shot pattern is thrown outwards by centrifugal force, resulting in an almost circular very open pattern- with a hole in the middle. For a visual, look at a donut. The slugster designation USUALLY means a rifled barrel, and rifle type sights, intended for use with sabot type slugs, not rifled slugs.
What we have heah is.... failure to communicate.