Pine Marten,
I bet you will find a range at which your drilling will shoot slugs from both barrels, and a rifle bullet, to the same spot...probably about 25 meters yards.
I have performed a little experimentation with my Sauer combi, and it seems to shoot the 173-gr 7x57R Sellier & Bellot and a Federal 3/4 ounce 20 gauge slug to the same spot about 30 yards. I have not yet tried further to tabulate the gaps, and deviation from the sights ( I have two flip up leaves, and I bet the higher one is for the slug).