I have used a lot of different types of slugs in a smoothbore, 24" barreled double gun. I have good results to 40-50 metres. In my opinion, the matter of having rifled chokes seems to be a better idea than a fully-rifled barrel, as the gun would still be usefull for a variety of conditions.
Where I live, people shoot game up to water buffalo with Brennekes with great success, although I have not done that myself. Hunters also like to load a slug in one barrel and buckshot in the other, sort of a poor man's cape gun. So I think that was what Fabarm had in mind when they designed their gun with a fixed rifled choke in one side and interchangeable chokes in the other.

Fred