A smooth bore is classified as a "shotgun" regardless of length, that is why all handgun barrels (aside from black powder, which are exempted) are all rifled. A smooth-bore pistol barrel is technically an illegal short-barreled shotgun.
A shotgun barrel is almost never rifled because a rifled barrel will spin the load of shot, and the centrifugal force will cause the shot pattern to go into a donut shape.
People who have tried shooting pigeons with .22 shot pellets will drive themselves crazy, because if the bird is at the point of aim, he will likely emerge unscathed.
Slugs may have rifled sabots which would cause the slug to spin down the barrel, but the barrel itself is smooth.