Good advice from all. This from the Benelli FAQ webpage:
Which chokes are recommended for slugs?
These chokes are recommended for slugs:
4 notches = Improved cylinder
5 notches = Cylinder
A cylinder choke is recommended for shooting rifled slugs in a smooth-bore barrel. Sabot slugs should only be shot through our fully-rifled slug barrels.
It is not safe to shoot slugs through chokes tighter than those listed here.