Rifled slugs are for smooth barrels.
That statement is not 100% correct. You can use rifled slugs in full rifling barrels. This should give better performance and better accuracy.
You have to use sabot in rifled barrels , since the sabot carrier has no rifling.
Rem, I have a Benelli also , (M1 20" smooth bore).For me I pick up all of the various rifled slugs available ( remington, federal, winchester, Fiocchi,Brenneke , etc...) I found the best combination have been the Brenneke M&P and the Federal classic foster slugs. Best advice I can give is to withdraw a few $$bills and buy everything you can get and try them out. The Remington solid copper, Federal Barnes eXpander,Winchster and lighfield Sabot are more accurate out to greater distance and cost more. If you are shooting around 50yrds or less any of the regular el cheapo slugs are adequate with good shot placement.