There's no such thing as a perfect sphere in lead roundballs. One side may have a dent or a wrinkle that the other side does not have. By spinning the ball, the effect of these imperfections average out as the ball flies through the air.
The knuckleball pitch analogy is a good one. If a baseball didn't have stitching and was a perfectly smooth sphere, then throwing it without spin wouldn't cause the erratic flight that a knuckleball is known for.
Some smoothbore shooters have experimented with dimpling their roundballs similar to a golf ball by rolling them between two course rasps and have claimed good results.
If a non spherical projectile has its center of mass ahead of its aerodynamic center of drag, it will fly pointed end first even without rifling. Examples of such projectiles include arrows, darts, badminton shuttlecocks, hollow base air rifle pellets, shotgun slugs, and some Minié ball designs. Yet they are also more accurate if they spin a little.
Last edited by B.L.E.; April 13, 2010 at 06:21 AM.