If you mean the spiral grooves inside of a rifle barrel- those are the lands (high spots) and grooves (low spots) that make up rifling. The bullet, being softer than the steel of the barrel, is pushed into the rifling when fired. The lands cut into the outer edge of the bullet, gripping it, and causes the bullet to rotate with the spiral. This rotation causes the bullet to travel in a straight line as it passes through the air- and is much more accurate than a smoothbore (no rifling) barrel. Ever notice how a football spins when a pass is thrown? Same thing.
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