Grooves cut into the inside of the barrel to apply spin to the projectile to stabilise its flight. Rifling is either made from straight grooves, or a polygon which curves down the barrel in a helical pattern. Some barrels have even rifling, some have a gradual increase or decrease in twist. The Walther PP, for instance, has tighter rifling near the chamber.

The rifling is measured with a number called the twist rate, which denotes how many complete loops of the inside of the barrel one inch of barrel contains.