Twist rate

In firearms, rifling consists of helical grooves in the internal (bore) surface of a gun's barrel, which impart a spin to a projectile around its long axis. This spin serves to gyroscopically stabilize the projectile, improving its aerodynamic stability and accuracy.

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.

The combination of length, weight and shape of a projectile determines the twist rate needed to stabilize it – barrels intended for short, large-diameter projectiles like spherical lead balls require a very low twist rate, such as 1 turn in 48 inches (122 cm). Barrels intended for long, small-diameter bullets, 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 have twist rates that increase down the length of the barrel, called a gain twist or progressive twist; a twist rate that decreases from breech to muzzle is undesirable, as it cannot reliably stabilize the bullet as it travels down the bore. Extremely long projectiles such as flechettes may require high twist rates; these projectiles must be inherently stable, and are often fired from a smoothbore barrel.

Muskets were smoothbore, large caliber weapons using ball-shaped ammunition fired at relatively low velocity. Due to the high cost and great difficulty of precision manufacturing, and the need to load readily from the muzzle, the musket ball was a loose fit in the barrel. Consequently, on firing the ball bounced off the sides of the barrel when fired and the final direction on leaving the muzzle was unpredictable.

Barrel rifling was invented in Augsburg, Germany in 1498. In 1520 August Kotter, an armourer of Nuremberg, Germany improved upon this work. Though true rifling dates from the mid-16th century, it did not become commonplace until the nineteenth century.

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