[arcticap]

Factory swagged round balls are relatively perfectly made though.
And when wrapped with a patch they are cushioned as well.
They don't seem to get upset much at all during ignition so any imperfection may actually be negligible. Although if they do indeed become upset and that's the case then there's no sense arguing whether a perfectly round ball can be fired from a gun barrel accurately without being deformed at all unless it's a low velocity airgun and then what's the point? So then rifling would be required for best performance as current practices suggest and the question is very hypothetical.
What if a perfect lead round ball had dimples on it like a golf ball that would allow it to fly with much less air turbulence? Then it would be the smooth surface of the ball that would better define its imperfection and not that its shape was being out of round.
However I really believe that it's more than merely the imperfection of its roundness that allows the rifling to better stabilize it over a longer distance than if it were a more perfect ball being fired through a smooth bore.
It could be argued that there's really no such thing as a perfectly calm day without any crosswind.
It's the rotational stabilization of the round ball due to rifling that should help to negate the Magnus effect and the wind.
If someone really wanted to try to answer the OP, why not just shoot some very perfectly round hard steel or hard lead alloy balls and see how much better they shoot out of a smooth bore than "imperfect" pure lead round balls, to try to better maximize the theoretical accuracy of round balls without the use of any rifling?
If they shoot better without rifling then maybe there's a discovery to be made, or maybe there isn't.