Answer:
Spiral grooves are etched or cut into the inside of the barrel. These grooves are called rifling. As a lead bullet moves down the barrel, it heats and expands slightly. The edges of the bullet catch in the grooves and the bullet spins.
The spinning motion helps the bullet to maintain a straighter course. A projectile that does not spin will begin to curve or tumble, wasting energy and moving off course. The spinning projectile keeps the nose end in front so that the bullet stays close to the target.
You can actually see the effect of the spinning by firing tracer bullets at a target at least 100 yards away. The dye on the surface of the bullet leaves a visible spiral in the air which can be observed for a few moments if the light is right.