When it comes to the old pumpkin ball rifled slug, I like the federal true ball. It is full-size and has been reliably accurate for me back in the day For penetration though you cannot beat the Brenkee. If you want the most penetration, then go with the 3 inch, I shot a bear with one and it was just sick.
The rifling on a rifled slug is nothing more than something to allow it to squeeze through a tight choke.
if you have a barrel with a choke tube, the rifle tube will improve the accuracy of the rifled or Brenkee slug. It probably does not impart enough spin to work well with sabot slugs.
those are my opinions, your mileage may vary