Get the bullet spinning, and different forces come into play - forces like angular momentum. If the bullet is spinning fast enough, it will stop behaving like a a stationary oblong object and start behaving more like a top. Nudge a stationary object and it'll fall over. Nudge a spinning top and it won't. It often won't even be nudged out of place. Instead, it will describe a little circle with its tip. This is called precessing. Precession is what a spinning bullet does as it moves through the air. Instead of cartwheeling end over end, it keeps going along the path on which it was shot, while the tip is just tooling around in a little circle. This is less than ideal. And ideal bullet it perfectly aligned with its course. But when things get less than ideal — and they frequently do — the spin keeps the bullet going along its course with nothing more than a slight wobble. This difference, a spinning bullet, allowed a long shot that was ten times more accurate than previous smoothbore guns.

Whether or not accurate guns were a boon is up for debate. But it's amazing how one simple change in a design allows for such a massive change in performance.

Via University of Houston and Schuemann