The use of a rifled tube on a smoothbore barrel for shooting slugs is an "on-again-off again" sort of thing. I have seen some such guns that were every bit as accurate as any fully-rifled barrel, and some that would not shoot at all. I think it is certainly worth a try, but you may find it necessary to try quite a few different slug loads before you find a satisfactory one. I tend to think a load that uses a pretty tough sabot might work best. This is because the projectile has reached a pretty high speed by the time it slams into the rifled portion of the bore, and the sabot is subjected to considerable stress as the rifling tries to bite into it and start it spinning.