The spinning of bullets fired from rifles and handguns is caused by a feature called rifling inside the barrel. Rifling consists of spiral grooves cut or formed into the interior surface of the gun barrel.
When a bullet travels through a rifled barrel, these spiral grooves engage with the bullet's surface, imparting a spin to the projectile as it moves forward. This works similarly to how the spiral grooves on a football make it spin when thrown properly.
This spinning motion serves a crucial purpose: it provides gyroscopic stabilization to the bullet during flight. Just as a spinning top remains upright, a spinning bullet maintains a consistent orientation as it travels through the air. This significantly improves accuracy by preventing the bullet from tumbling or veering off course due to air resistance and other forces.
The rate of spin depends on the "twist rate" of the rifling, typically expressed as one complete turn in a certain number of inches (like 1:9, meaning one complete rotation every nine inches of barrel). Different bullets perform best with different twist rates based on their weight, length, and intended use.