Herman,
The rifled chokes don't seem to work very well. Basically it's trying to spin the slug in about 3-4" compared to 20" or so with a fully rifled barrel. If you want to try slugs, use the modified choke, and the Fosters-style slugs. Not the newer sabot slugs. The Fosters slugs are rifled themselves and create enough spin in a smooth bore to be accurate enough out to 75-100 yards.

My personal slug gun is a Rem 870 smoothbore, modified fixed choke, Tasco WC2-7x32 mounted on a B-Square mount. My gun seems to like Winchester Super X 1oz slugs.