Just to clarify my earlier post: check the ballistics for both trajectory and Energy for "regular" slugs vs. sabot slugs. The sabot slugs, since they are much lighter, get much greater velocity and hence more energy. The old style solid lead slugs are so ballistically inefficient, and start out slow to begin with, that the kinetic energy drops off very quickly, compared to the sabot slugs. A 20 gauge with sabots is more like a ML than a smoothbore shotgun/rifled slug combo.
I personally haven't used a shotgun since I bought my in-line ML. And I used my old percussion round-ball rifle more than my 12 gauge smoothbore.