Where indeed is the question.

You need to shoot whatever you can get your hands on and just see for yourself.
If those smoothbore barrels are of the -shotgun- portion of your drilling, it's doubtful that the manufacturer regulated that gun for slugs to hit to the rifle sights.

Look at the proofs. Chances are if it was proofed -and regulated- for solids to hit to the sights out of the smooth bore barrels, it'll say so on the barrel flats.
Drillings are often made light, or light as they reasonably can be made in order to get the gun to handle more like a double and less like a crowbar. If the gun was proofed for loads on the lighter end of the spectrum, it might not be such a good idea to shoot slugs out of it.

--Tinker