Another important consideration that I don't know the answer to is "how much spin does a round projectile have when fired from a smoothbore, and what direction is the spin?"
If a ball fired from a smooth bore has some amount of random spin just from imperfections in the ball/bore/etcetera, then that might cause problems from the Magnus effect or similar aerodynamic effects.
Spinning the ball around an axis parallel to the direction of flight (like the spin imparted from a rifled barrel) won't cause a Magnus effect unless there's also a crosswind, but a ball spinning about either of the other two axes (pitching or yawing) will cause a Magnus effect even without a crosswind. For instance, if the ball has a backspin, it will experience an upward force as it travels downrange. If it has a topspin, the force will be downward.
It may be that the benefit of rifling with a spherical ball comes not from a stabilizing effect of the rolling spin, but from the fact that it gives the ball a predictable, repeatable spin, rather than a potentially random spin each time the rifle is fired.