If it's a short smoothbore barrel, it may look tapered on the outside because the barrel wall may be thicker at the rear than it is at the muzzle. I would expect the initial pressure in the chamber and the breech end of the barrel to be higher than it is at the muzzle end, since the expanding gases are confined to a smaller space back there than they are by the time they've shoved the projectile all the way to the muzzle of the barrel.
Your barrel's internal diameter is probably not tapered the way its outer diameter is. It's probably a cylinder bore barrel, and you can probably fire rifled (Foster) slugs out of it with no problems.
The "rifling" on a lead rifled slug isn't really meant to impart spin to the slug. Its primary function is to allow the slug to deform as it passes through a choke tube (but this does not make the slug safe to fire through a very restrictive choke). Even if it did have a mild to moderate choke, you could safely fire rifled slugs through it, but the more constricted the choke is, the less accuracy you can expect out of a rifled slug.
You should probably NOT fire a slug out of any barrel with a very restrictive choke (full, for example); nor should you fire a slug out of an overbored barrel.
Unless your barrel is rifled, do not use sabot slugs.
