The Brenneke Classic slug was put together with a screw:
As far as I know the Brenneke Classic slug is still made the same. They also have "modern" slugs with plastic tailwads that insert into the slugs.
Here is a pic of my Brenneke'ized Lee slugm from a modified Lee mould:
Since these are wad slugs I use hot melt glue cast in a form then screw to the slug. Lately I have found that I can cast directly onto the slug but the drive key has to be burred or a screw installed with the head sticking out for the glue to grip or they often do not bond to the slug and come of in flight.
Most of the new style Russian slugs use proprietary "plug in" tailwads. Some Russian slugs use a screw from the nose into the top deck of a brush wad and that seems to work well.
I believe that the key to Brenneke Classic slugs is that they either align the slug and wads in a form when screwed together or they attached oversize wads then shear to suit the slug after assembly. I have found it difficult to keep felt wads accurately aligned at assembly even when punched in jigs and assembled in a jig... things tend to shift and tilt just enough to throw out alignment. If pre-assembled then sheared to final size that should be better. Think cast bullet and what you would reject if not perfectly cast. Might not be quite that critical for smoothbore shotgun but it really doesn't take much error to ruin accuracy.
Anyway, I digress.
A solid slug isn't going to be accurate from a smoothbore unless it is drag stabilized so a tailwad is a must. If hollow base and weight forward enough like a Foster slug it should work from smoothbore. Fit to bore has to be tight as well. Any rattle fit will give poor accuracy.
If solid and tight enough fit into wad/bore the lead slug should shoot okay from a rifled barrel. With the multi point surface it may not grip rifling well. Testing will tell you. If you get keyholes or poor accuracy then fit is likely not good.
Just my thoughts.
Longbow