Rifled barrels were originally designed to extend the range of foster style slugs. Sabots are relatively new in the scheme of things.

Rifled slugs out of a smooth bore don't usually give great accuracy and have poor ballistics. When shooting a smooth bore you hear things like hitting a pie plate at a certain distance. This isn't exactly precision shooting. And then consider most are just using a bead sight and that makes it even more difficult to shoot consistently.

It is hard to nail down "how much" better one is than the other because it really depends on how well you shoot, how well your gun shoots and how well it likes the load you are using. I can give you examples of what I have seen. With a smooth bore and some sort of sights, like clamp on fiber optics or even a low powered scope I have seen groups as good as 3 or 4 inches at 100 yards. This is for sure not the norm though. I would say the norm is probably closer to 4 or 5 inch groups from 50 to 80 yards depending on your gun and set up.

Now with a rifled barrel you are looking at touching holes at 100 yards with a good set up. I would consider 4 inch groups at 100 yards with a rifled barrel and a scope to be poor. Usable no doubt, but you could do better in my opinion if you shoot well enough. Average would probably be 2 or 3 inch groups at 100 yards and MOA if you have a good gun and find the right load.

And you can shoot rifled slugs out of a rifled barrel, it just makes a mess. Doesn't hurt anything though. And the rifling doesn't matter since it doesn't actually impart any spin onto the bullet. This is a myth, and I don't know why they call them "rifled" bullets. The fins are there to crush as they go through the choke since not all shotgun barrels are consistent on size and choke. This is why shooting a tight choke with slugs isn't very accurate, it really deforms the slug when it goes through and messes up the flight.

The reason foster (rifled) slugs fly as well as they do is because they are hollow and have the weight forward like a shuttle cock in bad mitten.

My H&R ultra slug gun will shoot a few brands of cheap rifled slugs VERY well until the barrel fouls out, which is about 4 shots. I am talking one large ragged hole at 100 yards. Maybe 2 inch groups or so. And I get 1.5 to 2 inch groups out of it with Winchester Platinum sabots. The sabots cost around 15 bucks for 5 rounds and the foster slugs cost about 3 bucks.

I have buddies that own 870's, 1100's and 1187's with cantilever rifled barrels and they get similar accuracy when using the right load.

And in my opinion an inline muzzle loader is even more accurate and easier to work up a load for. My Remington 700ML will shoot 1 inch groups at 100 yards and around 3 inch groups at 150 which is my max distance for that gun. But I have seen some that are better.

Paul