What causes bullets to spin when shot from a rifle or handgun?

Bullets spin when shot from a rifle or handgun primarily because of rifling, which refers to the spiral grooves cut into the barrel of the firearm. This design causes the bullet to engage with the grooves as it travels down the barrel. When the bullet is fired, it is pushed out of the barrel by expanding gases from the ignited gunpowder. The rifling imparts a rotational force on the bullet, causing it to spin around its longitudinal axis.

This spinning motion is crucial for stabilizing the bullet in flight, allowing for greater accuracy and distance. Without rifling, bullets would travel without any spin, making them less stable and more prone to tumbling in the air, which would significantly reduce their effectiveness and precision. The other options, while related to the mechanics of a firearm, do not directly contribute to the spinning of the bullet when it is fired.