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:). 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.; Petzal, David E., and Bourjaily, Phil, with Fenson, Brad. The Total Gun Manual (Canadian edition) (San Francisco: WeldonOwen, 2014), p. 5. 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 , or slightly longer, although for a typical multi-purpose muzzleloader rifle, a twist rate of 1 in 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 or 32 calibers twist. Civilian AR-15 rifles are commonly found in 1 in (54.8 cal) for older rifles and 1 in (41.1 cal) for most newer rifles. However, some are made with 1 in 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:
where is the twist rate expressed in bore diameters; is the twist length required to complete one full projectile revolution (in mm or in); and is the bore diameter (diameter of the lands, in mm or in).
The twist travel and the bore diameter must be expressed in a consistent unit of measure, i.e. metric (mm) or imperial (in).
The third method simply reports the angle of the grooves relative to the bore axis, measured in degrees.
The latter two methods have the inherent advantage of expressing twist rate as a ratio and provide an easy way to understand whether a twist rate is relatively slow or fast, even when comparing bores of differing diameters.
In 1879, George Greenhill, a professor of mathematics at the Royal Military Academy (RMA) at Woolwich, London, UK developed a rule of thumb for calculating the optimal twist rate for lead-core bullets. This shortcut uses the bullet's length and requires no allowances for weight or nose shape The eponymous Greenhill Formula, still used today, is:
where is 150 (use 180 for muzzle velocities higher than 2,800 f/s); is the bullet's diameter in inches; is the bullet's length in inches; and is the bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation).
The original value of was 150, which yields a twist rate in inches per turn, when given the diameter and the length of the bullet in inches. This works up to velocities of about 840 m/s (2800 ft/s); above those velocities, a of 180 should be used. For instance, with a velocity of 600 m/s (2000 ft/s), a diameter of and a length of , the Greenhill formula would give a value of 25, which means 1 turn in .
Improved formulas for determining stability and twist rates include the Miller Twist Rule and the McGyro program developed by Bill Davis and Robert McCoy.
If an insufficient twist rate is used, the bullet will begin to yaw and then tumble; this is usually seen as "keyholing", where bullets leave elongated holes in the target as they strike at an angle. Once the bullet starts to yaw, any hope of accuracy is lost, as the bullet will begin to veer off in random directions as it precesses.
Conversely, a rate of twist that is too high can also cause problems. The excessive twist can cause accelerated barrel wear and, coupled with high velocities, induce a very high spin rate, which can lead to projectile jacket ruptures, causing high-velocity spin-stabilized projectiles to disintegrate in flight. Projectiles made of a single metal cannot practically achieve flight and spin velocities high enough to cause disintegration in flight. Smokeless powder can produce muzzle velocities of approximately for spin stabilized projectiles and more advanced propellants used in smoothbore tank guns can produce muzzle velocities of approximately . A higher twist than needed can also cause more subtle problems with accuracy; any inconsistency within the bullet, such as a void that causes an unequal distribution of mass, may be magnified by the spin. Undersized bullets also have problems, as they may not enter the rifling exactly concentric and coaxial to the bore, and excess twist will exacerbate the accuracy problems this causes.
A bullet fired from a rifled barrel can spin at over 300,000 rpm (5 kHz), depending on the bullet's muzzle velocity and the barrel's twist rate.
The general definition of the spin of an object rotating around a single axis can be written as:
where is the linear velocity of a point in the rotating object (in units of distance/time) and refers to the circumference of the circle that this measuring point performs around the axis of rotation.
A bullet that matches the rifling of the firing barrel will exit that barrel with a spin:
where is the muzzle velocity and is the twist rate.
For example, an M4 Carbine with a twist rate of 1 in and a muzzle velocity of will give the bullet a spin of 930 m/s / 0.1778 m ≈ 5.230 kHz (313,835 rpm).
Excessive rotational speed can exceed the bullet's designed limits, and the resulting centrifugal force can cause the bullet to disintegrate radially during flight.
Design A barrel of circular bore cross-section is not capable of imparting a spin to a projectile, so a rifled barrel has a non-circular cross-section. Typically, the rifled barrel contains one or more grooves that run down its length, giving it a cross-section resembling an internal gear. However, it can also take the shape of a polygon, usually with rounded corners. Since the barrel is not circular in cross-section, it cannot be accurately described with a single diameter. Rifled bores may be described by the bore diameter or land diameter (the diameter between lands or high points in the rifling), or by groove diameter (the diameter between grooves or low points in the rifling). Differences in naming conventions for cartridges can confuse; for example, the projectiles of the .303 British are actually slightly larger in diameter than the projectiles of the .308 Winchester, because the ".303" refers to the bore diameter in inches (bullet is .312), while the ".308" refers to the bullet diameter in inches (7.92 mm and 7.82 mm, respectively).
Despite differences in form, the common goal of rifling is to deliver the projectile accurately to the target. In addition to imparting spin to the bullet, the barrel must hold the projectile securely and concentrically as it travels down the barrel. This requires that the rifling meet several tasks: It must be sized so that the projectile will swage or obturate upon firing to fill the bore. The diameter should be consistent, and must not increase towards the muzzle. The rifling should be consistent down the length of the bore, without changes in cross-section, such as variations in groove width or spacing. It should be smooth, with no scratches lying perpendicular to the bore, so it does not abrade material from the projectile. The chamber and crown must smoothly transition the projectile into and out of the rifling.