Rifling is the term for helical grooves machined into the internal surface of a firearm's barrel for imparting a spin to the fired projectile to improve its aerodynamic stability and accuracy. It is also the term (as a verb) for creating such grooves. Firearms without rifling are called smoothbore. Rifling is measured by twist rate, the distance the rifling takes to complete one full revolution, expressed as a ratio with 1 as its base (e.g., 1:10 inches (25.4 cm)). A shorter distance/lower ratio indicates a tighter twist, generating a higher spin rate (and greater projectile stability). 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. Rifling that increases the twist rate from breech to muzzle is called a gain or progressive twist; a rate which decreases down the length of a barrel 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; it is often stabilized aerodynamically instead. An aerodynamically stabilized projectile can be fired from a smoothbore barrel without a reduction in accuracy.
History
Muskets are smoothbore, large caliber weapons using ball-shaped ammunition fired at relatively low velocity. Due to the high cost, great difficulty of precision manufacturing, and the need to load readily and speedily from the muzzle, musket balls were generally a loose fit in the barrels. Consequently, the balls would often bounce off the sides of the barrel, and their final destination after leaving the muzzle was less predictable. This was countered when accuracy was more important, for example, when hunting, by using a tighter-fitting combination of a closer-to-bore-sized ball and a patch. The accuracy was improved, but it is still not reliable for precision shooting over long distances. Like the inventor of gunpowder itself, the inventor of barrel rifling is not yet definitively known. Straight grooving had been applied to small arms since at least 1480, originally intended as "soot grooves" to collect gunpowder residue. Some of the earliest recorded European attempts at spiral-grooved musket barrels were by Gaspard Kollner, a gunsmith of Vienna in 1498, and by Augustus Kotter of Nuremberg in 1520. Some scholars allege that, at the end of the 15th century, Kollner's works used only straight grooves, and that it was not until he received help from Kotter that a working spiral-grooved firearm was made. There may have been attempts even earlier than this, as the main inspiration of rifled firearms came from archers and crossbowmen who realized that their projectiles flew far faster and more accurately when they imparted rotation through twisted fletchings. Though true rifling dates from the 16th century, it had to be engraved by hand and consequently did not become commonplace until the mid-19th century. Due to the laborious and expensive manufacturing process involved, early rifled firearms were primarily used by wealthy recreational hunters, who did not need to fire their weapons many times in rapid succession and appreciated the increased accuracy. Rifled firearms were not popular with military users since they were difficult to clean, and loading projectiles presented numerous challenges. If the bullet was of sufficient diameter to take up the rifling, a large mallet was required to force it down the bore. If, on the other hand, it was of reduced diameter to assist in its insertion, the bullet would not fully engage the rifling, and accuracy would be reduced. The first practical military weapons to use rifling with black powder were breech-loaders, such as the Queen Anne pistol.
Twist rate
For best performance, the barrel should have a twist rate sufficient to spin-stabilize any bullet that it would reasonably be expected to fire, but not significantly more. Large-diameter bullets provide more stability, as the larger radius provides more gyroscopic inertia. In contrast, long bullets are harder to stabilize, as they tend to be very back-heavy and the aerodynamic pressures have a longer arm ("lever") to act on. The slowest twist rates are found in muzzle-loading firearms meant to fire a round ball; these will have twist rates as low as 1 in 72 inches (180 cm), or slightly longer, although for a typical multi-purpose muzzleloader rifle, a twist rate of 1 in 48 inches (120 cm) is very common. The M16A2 rifle, which is designed to fire the 5.56×45mm NATO SS109 ball and L110 tracer bullets, has a 1 in 7-inch (18 cm) or 32 calibers twist. Civilian AR-15 rifles are commonly found in 1 in 12 inches (30 cm) (54.8 cal) for older rifles and 1 in 9 inches (23 cm) (41.1 cal) for most newer rifles. However, some are made with 1 in 7 inches (18 cm) or 32-caliber twist rates, the same as those used for the M16 rifle. Rifles, which generally fire longer, smaller diameter bullets, will in general have higher twist rates than handguns, which fire shorter, larger diameter bullets. There are three methods in use to describe the twist rate: Traditionally, the most common method expresses the twist rate in terms of the 'travel' (length) required to complete one full revolution of the projectile in the rifled barrel. This method does not provide a straightforward understanding of whether a twist rate is relatively slow or fast when comparing bores of different diameters. The second method describes the 'rifled travel' required to complete one full projectile revolution in calibers or bore diameters:
twist = L D bore , {\displaystyle {\text{twist}}={\frac {L}{D_{\text{bore}}}},}
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