In firearms, rifling is machining helical grooves into the internal (bore) surface of a gun's barrel for the purpose of exerting torque and thus imparting a spin to a projectile around its longitudinal axis during shooting to stabilize the projectile longitudinally by conservation of angular momentum, improving its aerodynamic stability and accuracy over smoothbore designs.

Rifling is characterized by its twist rate, which indicates the distance the rifling takes to complete one full revolution, such as "1 turn in 10 inches" (1:10 inches), "1 turn in 254 mm" ("1:254 mm" or "1:25.4 cm)", or the like. Normally, an experienced shooter can infer the units of measurement from the numbers alone. A shorter distance indicates a faster twist, meaning that for a given velocity the projectile will rotate at a higher spin rate. The combination of length, weight, and shape of a projectile determines the twist rate needed to gyroscopically stabilize it: barrels intended for short, large-diameter projectiles such as spherical lead balls require a very low twist rate, such as 1 turn in 48 inches (122 cm). Barrels intended for long, small-diameter projectiles, such as the ultra-low-drag 80-grain 0.223 inch bullets (5.2 g, 5.56 mm), use twist rates of 1 turn in 8 inches (20 cm) or faster. In some cases, rifling will increase the twist rate as the projectile travels down the length of the barrel, called a gain twist or progressive twist; a twist rate that decreases from breech to muzzle is undesirable because it cannot reliably stabilize the projectile as it travels down the bore.

An extremely long projectile, such as a flechette, requires impractically high twist rates to stabilize; they are often stabilized aerodynamically instead. An aerodynamically stabilized projectile can be fired from a smoothbore barrel without a reduction in accuracy.