The laws of physics can be used to understand how a bullet moves after it leaves the barrel. The way the bullet behaves after it leaves the barrel is called 'external ballistics'. The main factors affecting the aerodynamics of a bullet in flight are the bullet's shape and the rotation from the rifling of the gun barrel. Spinning helps stabilize the bullet gyroscopically and aerodynamically (spinning keeps it on track). Any small differences in the shape of the bullet are mostly canceled as it spins. The first guns did not have rifling. With smoothbore firearms (no rifling), a round bullet was best because it was the same in any position. But these bullets were not very stable. They were also not very accurate. Still, the round shape of bullets did not change very much for hundreds of years.