obturation - swaging
I've learned that many prefer to ram a round ball into the chamber of a revolver so that it leaves a ring of lead shaved off of the ball.
In order to do this one has to put a ball in the cylinder chamber that is a few thousand's of an inch to large for the chamber.
When you do this it changes the shape of the ball however minutely. As the ball is no longer "perfectly round", (and it never was), it can benefit from being spun as it engages the rifling.
If the boolit is to engage the rifling, then it must be larger than the "inner" barrel diameter described by the surfaces of the lands. So in order to engage the grooves the shape of the round ball must once again be "swaged" to fit through the barrel.
Ideally, ball continues down the barrel and is tight as tick in the barrel so that the force behind it, the explosion, does not leak out around the sides.
So there is some squeezing, or swaging, of the soft round lead ball in the ramming and loading phase as well as the firing phase.
And all of the previous posters who have told of the need to impart spin to even a perfectly round object are purely correct.
When we take into account the shaving and swaging of a ball every bit of help stabilizing that "round ball" helps.
Also, there is a big, huge explosion happening on one end of that ball and that does tend to obturate the back of the ball and now I seem to remember some very intense discussions and threads I have read on this very subject.
Rifling is good, and some can shoot smoothbore with fascinating accuracy.