<P> In firearms, rifling is the helical groove pattern that is machined 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 . This spin serves to gyroscopically stabilize the projectile by conservation of angular momentum, improving its aerodynamic stability and accuracy over smoothbore designs . </P> <P> Rifling is often described 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), or "1 turn in 254 mm" (1: 254 mm). A shorter distance indicates a "faster" twist, meaning that for a given velocity the projectile will be rotating at a higher spin rate . </P>

Why is rifling present in the bore of the barrel