shotguns with rifled slug barrels, even among the same brand/model vary greatly on what ammo they like. You may have to by several brands/types of slugs or Sabot Slugs shoot well in your individual gun. Examples:
My mossberg 695 bolt action shoots best with 3" partition gold sabots. My mossberg 930 autoloader likes the 3" Brennekey Gold full bore slugs. My sons 20 guage Mossberg 500 with rifled barrel likes the Remington buckhammer.
off season sales can help - many gun stores like to gett the slugs out of inventory and have brand new ammo the next fall.