Rifling

Rifling of a 105 mmRoyal Ordnance L7tank gun

Conventional rifling of a 90 mm M75cannon(production year 1891,Austria-Hungary)

Rifling in aGAU-8autocannon

Riflingis the term forhelicalgroovesmachinedinto the internal surface of afirearms'sbarrelfor imparting aspinto aprojectileto improve itsaerodynamicstability and accuracy. It is also the term (as a verb) for creating such grooves.

Rifling is measured intwist 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 faster 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 togyroscopicallystabilize 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-grain0.223 inch bullets (5.2 g, 5.56 mm), use twist rates of 1 turn in 8 inches (20 cm) or faster.

Rifling which increases the twist rate frombreechtomuzzleis called againorprogressive 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 aflechette,requires impractically high twist rates to stabilize; it is often stabilized aerodynamically instead. An aerodynamically stabilized projectile can be fired from asmoothborebarrel without a reduction in accuracy.

History

Traditional rifling of a 9 mm handgun barrel

Main article:Rifle

Musketsaresmoothbore,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, on firing the balls would often bounce off the sides of the barrel when fired and the 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 still not reliable for precision shooting over long distances.

Like the invention of gunpowder itself, the inventor of barrel rifling is not yet definitely known. Straight grooving had been applied to small arms since at least 1480, originally intended as "soot grooves" to collectgunpowder residue.

Some of the earliest recorded European attempts of spiral-grooved musket barrels were ofGaspard Kollner,a gunsmith ofViennain 1498 and Augustus Kotter ofNurembergin 1520. Some scholars allege that Kollner's works at the end of the 15th century only used straight grooves, and 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 was reduced.

The first practical military weapons using rifling with black powder werebreech loaderssuch as theQueen Anne pistol.

Twist rate

Russian 122 mm shrapnel shell (which has been fired) showing rifling marks on thecopperalloydriving bandaround its base, indicating clockwise spin

Cannonballequipped with winglets inside the bore of a rifled cannon c. 1860

Ogival shell of theLa Hitte system,1858, designed to engage with clockwise rifling

For best performance, the barrel should have a twist rate sufficient to spin stabilize anybulletthat it would reasonably be expected to fire, but not significantly more. Large diameter bullets provide more stability, as the larger radius provides moregyroscopic inertia,while long bullets are harder to stabilize, as they tend to be very backheavy and the aerodynamic pressures have a longer arm ( "lever" ) to act on. The slowest twist rates are found inmuzzle-loadingfirearms 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. TheM16A2rifle, which is designed to fire the5.56×45mm NATO SS109 ball and L110 tracerbullets, has a 1 in 7-inch (18 cm) or 32 calibers twist. CivilianAR-15rifles are commonly found with 1 in 12 inches (30 cm) or 54.8 calibers for older rifles and 1 in 9 inches (23 cm) or 41.1 calibers for most newer rifles, although some are made with 1 in 7 inches (18 cm) or 32 calibers twist rates, the same as 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:

The, traditionally speaking, most common method expresses the twist rate in terms of the 'travel' (length) required to complete one full projectile revolution in the rifled barrel. This method does not give an easy or straightforward understanding of whether a twist rate isrelativelyslow or fast when bores of different diameters are compared.

The second method describes the 'rifled travel' required to complete one full projectile revolution in calibers or bore diameters:

$${\text{twist}}={\frac {L}{D_{\text{bore}}}},$$

where${\text{twist}}$is the twist rate expressed in bore diameters;$L$is the twist length required to complete one full projectile revolution (in mm or in); and$D_{\text{bore}}$is the bore diameter (diameter of the lands, in mm or in).

The twist travel$L$and the bore diameter$D_{\text{bore}}$must be expressed in a consistent unit of measure, i.e. metric (mm)orimperial (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 give an easy understanding if a twist rate isrelativelyslow or fast even when comparing bores of differing diameters.

In 1879,George Greenhill,a professor of mathematics at theRoyal Military Academy (RMA) at Woolwich,London, UKdeveloped arule of thumbfor calculating the optimal twist rate for lead-core bullets. This shortcut uses the bullet's length, needing no allowances for weight or nose shape.The eponymousGreenhill Formula,still used today, is:

$${\text{twist}}={\frac {CD^{2}}{L}}\times {\sqrt {\frac {\mathrm {SG} }{10.9}}}$$

where$C$is 150 (use 180 for muzzle velocities higher than 2,800 f/s);$D$is the bullet's diameter in inches;$L$is the bullet's length in inches; and$\mathrm {SG}$is the bullet'sspecific gravity(10.9 for lead-core bullets, which cancels out the second half of the equation).

The original value of$C$was 150, which yields a twist rate in inches per turn, when given the diameter$D$and the length$L$of the bullet in inches. This works to velocities of about 840 m/s (2800 ft/s); above those velocities, a$C$of 180 should be used. For instance, with a velocity of 600 m/s (2000 ft/s), a diameter of 0.5 inches (13 mm) and a length of 1.5 inches (38 mm), the Greenhill formula would give a value of 25, which means 1 turn in 25 inches (640 mm).

Improved formulas for determining stability and twist rates include theMiller Twist Ruleand the McGyro programdeveloped by Bill Davis and Robert McCoy.

AParrott rifle,used by bothConfederateandUnionforces in theAmerican Civil War.

If an insufficient twist rate is used, the bullet will begin toyawand 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 itprecesses.

Conversely, too high a rate of twist can also cause problems. The excessive twist can cause accelerated barrel wear, and coupled with high velocities also induce a very high spin rate which can cause projectilejacketruptures causing high velocity spin stabilized projectiles to disintegrate in flight. Projectiles made out of mono metals cannot practically achieve flight and spin velocities such that they disintegrate in flight due to their spin rate.Smokeless powdercan produce muzzle velocities of approximately 1,600 m/s (5,200 ft/s) for spin stabilized projectiles and more advanced propellants used insmoothboretank guns can produce muzzle velocities of approximately 1,800 m/s (5,900 ft/s).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 exactlyconcentricandcoaxialto the bore, and excess twist will exacerbate the accuracy problems this causes.

A bullet fired from a rifled barrel can spin at over 300,000rpm(5kHz), depending on the bullet'smuzzle velocityand the barrel'stwist rate.

The general definition of the spin$S$of an object rotating around a single axis can be written as:

$$S={\frac {\upsilon }{C}}$$

where$\upsilon$is the linearvelocityof a point in the rotating object (in units of distance/time) and$C$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:

$$S={\frac {\upsilon _{0}}{L}}$$ where$\upsilon _{0}$is the muzzle velocity and$L$is the twist rate.

For example, an M4 Carbine with a twist rate of 1 in 7 inches (177.8 mm) and a muzzle velocity of 3,050 feet per second (930 m/s) will give the bullet a spin of 930 m/s / 0.1778 m = 5.2 kHz (314,000 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 circularborecross-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 aninternal gear,though it can also take the shape of apolygon,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 thebore diameter(the diameter across thelandsor high points in the rifling), or bygroove diameter(the diameter across thegroovesor low points in the rifling). Differences in naming conventions forcartridgescan cause confusion; for example, the projectiles of the.303 Britishare 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 the 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 a number of tasks:

• It must be sized so that the projectile willswageorobturateupon 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.

Rifling may not begin immediately forward of the chamber. There may be an unrifled throat ahead of the chamber so a cartridge may be chambered without pushing the bullet into the rifling. This reduces the force required to load a cartridge into the chamber, and prevents leaving a bullet stuck in the rifling when an unfired cartridge is removed from the chamber. The specified diameter of the throat may be somewhat greater than groove diameter, and may be enlarged by use if hot powder gas melts the interior barrel surface when the rifle is fired.Freeboreis a groove-diameter length ofsmoothborebarrel without lands forward of the throat. Freebore allows the bullet to transition fromstatic frictionto sliding friction and gain linearmomentumprior to encountering the resistance of increasing rotational momentum. Freebore may allow more effective use of propellants by reducing the initial pressure peak during the minimum volume phase ofinternal ballisticsbefore the bullet starts moving down the barrel. Barrels with freebore length exceeding the rifled length have been known by a variety of trade names includingparadox.

Manufacture

Rifling a75-mm gunbarrel during WWI

Further information:Internal ballistics

An early method of introducing rifling to a pre-drilled barrel was to use a cutter mounted on a square-section rod, accurately twisted into a spiral of the desired pitch, mounted in two fixed square-section holes. As the cutter was advanced through the barrel it twisted at a uniform rate governed by the pitch. The first cut was shallow. The cutter points were gradually expanded as repeated cuts were made. The blades were in slots in a woodendowelwhich were gradually packed out with slips of paper until the required depth was obtained. The process was finished off by casting aslugof molten lead into the barrel, withdrawing it and using it with a paste ofemeryand oil to smooth the bore.

Most rifling is created by either:

• Cutting one groove at a time with atool(cut riflingorsingle point cut rifling);
• Cutting all grooves in one pass with a special progressivebroachingbit (broached rifling);
• Pressingall grooves at once with a tool called a "button" that is pushed or pulled down the barrel (button rifling);
• Forgingthebarrelover amandrelcontaining a reverse image of the rifling, and often the chamber as well (hammer forging);
• Flow formingthebarrelpreform over amandrelcontaining a reverse image of the rifling (rifling by flow forming)
• Using non-contact forces such aschemical reactionorheatfromlasersource toetchthe rifling pattern (etching rifling)
• Machining the rifling grooves texture on a thin metal plate, then folding the plate into the inner bore of the barrel (liner rifling)

Thegroovesare the spaces that are cut out, and the resulting ridges are calledlands.These lands and grooves can vary in number, depth, shape, direction of twist (right or left), and twist rate. The spin imparted by rifling significantly improves the stability of the projectile, improving both range and accuracy. Typically rifling is a constant rate down the barrel, usually measured by the length of travel required to produce a single turn. Occasionally firearms are encountered with again twist,where the rate of spin increases from chamber to muzzle. While intentional gain twists are rare, due to manufacturing variance, a slight gain twist is in fact fairly common. Since a reduction in twist rate is very detrimental to accuracy,gunsmithswho aremachininga new barrel from a rifled blank will often measure the twist carefully so they may put the faster rate, no matter how minute the difference is, at the muzzle end.

Projectiles

57-N-231 standard7.62×39mmmilitary bullets with steel core - the one on the left is unfired, the one on the right is fired, with the rifling grooves visible. Notice the copper wash scraped off and the steel jacket is exposed on the groove marks

Three recovered7.62×51mm NATObullets (next to an unfired cartridge), showing rifling marks imparting anti-clockwise spin

The original firearms wereloaded from the muzzleby forcing a ball from the muzzle to the chamber. Whether using a rifled or smooth bore, a good fit was needed to seal the bore and provide the best possible accuracy from the gun. To ease the force required to load the projectile, these early guns used an undersized ball, and a patch made of cloth, paper, or leather to fill thewindage(the gap between the ball and the walls of the bore). The patch acted as awaddingand provided some degree ofpressure sealing,kept the ball seated on the charge ofblack powder,and kept the ball concentric to the bore. In rifled barrels, the patch also provided a means to transfer the spin from the rifling to the bullet, as the patch is engraved rather than the ball. Until the advent of the hollow-basedMinié ball,which expands and obturates upon firing to seal the bore and engage the rifling, the patch provided the best means of getting the projectile to engage the rifling.

Inbreech-loading firearms,the task of seating the projectile into the rifling is handled by thethroatof thechamber.Next is thefreebore,which is the portion of the throat down which the projectile travels before the rifling starts. The last section of the throat is thethroat angle,where the throat transitions into the rifled barrel.

The throat is usually sized slightly larger than the projectile, so the loaded cartridge can be inserted and removed easily, but the throat should be as close as practical to the groove diameter of the barrel. Upon firing, the projectile expands under the pressure from the chamber, and obturates to fit the throat. The bullet then travels down the throat and engages the rifling, where it is engraved, and begins to spin. Engraving the projectile requires a significant amount of force, and in some firearms there is a significant amount of freebore, which helps keep chamber pressures low by allowing the propellant gases to expand before being required to engrave the projectile. Minimizing freebore improves accuracy by decreasing the chance that a projectile will distort before entering the rifling.

When the projectile is swaged into the rifling, it takes on a mirror image of the rifling, as the lands push into the projectile in a process calledengraving.Engraving takes on not only the major features of the bore, such as the lands and grooves, but also minor features, like scratches and tool marks. The relationship between the bore characteristics and the engraving on the projectile are often used inforensic ballistics.

Recent developments

Conventional rifling (left) andpolygonal rifling(right). Both types of rifling use a spiraling pattern

The spiraling pattern (here with polygonal rifling) is shown

The grooves most commonly used in modern rifling have fairly sharp edges. More recently,polygonal rifling,a throwback to the earliest types of rifling, has become popular, especially inhandguns.Polygonal barrels tend to have longer service lives because the reduction of the sharp edges of the land (the grooves are the spaces that are cut out, and the resulting ridges are called lands) reduces erosion of the barrel. Supporters of polygonal rifling also claim higher velocities and greater accuracy. Polygonal rifling is currently seen on pistols fromCZ,Heckler & Koch,Glock,Tanfoglio,and theKahr Arms(P seriesonly), as well as theDesert Eagle.

For field artillery pieces, theextended range, full bore(ERFB) concept developed in early 1970s by Dennis Hyatt Jenkins and Luis PalacioofGerald Bull'sSpace Research Corporationfor theGC-45 howitzerreplaces thebourreletwith small nubs, which both tightly fit into lands of the barrel.Guns capable of firing these projectiles have achieved significant increases in range, but this is compensated with a significantly (3-4 times) decreased accuracy, due to which they were not adopted by NATO militaries.Unlike a shell narrower than the gun's bore with asabot,ERFB shells use the full bore, permitting a larger payload. Examples include the South AfricanG5and the GermanPzH 2000.ERFB may be combined withbase bleed.

Variable pitch rifling

Again-twistorprogressive riflingbegins with a slowtwist ratethat gradually increases down the bore, resulting in very little initial change in the projectile'sangular momentumduring the first few inches of bullet travel after it enters thethroat.This enables the bullet to remain essentially undisturbed and trued to the case mouth. After engaging the rifling at the throat, the bullet is progressively subjected toacceleratedangular momentumas it is propelled down the barrel. The theoretical advantage is that by gradually increasing the spin rate, torque is imparted along a much longer bore length, allowing thermomechanicalstressto be spread over a larger area rather than being focused predominantly at the throat, which typicallywearsout much faster than other parts of the barrel. Gain-twist rifling was used prior to and during theAmerican Civil War(1861–65). ColtArmyandNavyrevolvers both employed gain-twist rifling. Gain-twist rifling, however, is more difficult to produce than uniform rifling, and therefore is more expensive. The military has used gain-twist rifling in a variety of weapons such as the20 mmM61 VulcanGatling gun used in some current fighter jets and the larger30 mmGAU-8 AvengerGatling gun used in the A10 Thunderbolt II close air support jet. In these applications it allows lighter construction of the barrels by decreasing chamber pressures through the use of low initial twist rates but ensuring the projectiles have sufficient stability once they leave the barrel. It is seldom used in commercially available products, though notably on theSmith & Wesson Model 460(X-treme Velocity Revolver).

See also

• Paradox gun
• Rifled musket
• Comparison microscope
• Gun barrel sequence
• Greenhill formula
• Glossary of firearms terminology
• Miller twist rate

External links

• Articleon barrel making from anIHMSAshooter
• Articleon barrel making from Lilja, a maker of world class competition barrels
• Articleon making and measuring rifling by Lilja; includes pictures of button rifling machine
• ArticleArchived2016-06-22 at theWayback Machineon rifle barrel manufacturing process including gundrilling, reaming, rifling, and finishing
• 6mmBR articleon barrels
• Bore slugging tutorialArchived2006-06-18 at theWayback Machine,explaining now to determine the true bore and groove size and choose appropriate bullet diameters
• Calculating Bullet RPM — Spin Rates And Stability
• Common rifling dimensions of revolver, pistol and rifle ammunition