In
firearms, rifling is
machining helical
Helical may refer to:
* Helix, the mathematical concept for the shape
* Helical engine, a proposed spacecraft propulsion drive
* Helical spring, a coilspring
* Helical plc, a British property company, once a maker of steel bar stock
* Helicoil
A t ...
grooves into the internal (bore) surface of a
gun
A gun is a ranged weapon designed to use a shooting tube (gun barrel) to launch projectiles. The projectiles are typically solid, but can also be pressurized liquid (e.g. in water guns/cannons, spray guns for painting or pressure washing, p ...
's
barrel for the purpose of exerting
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
and thus imparting a
spin to a
projectile around its longitudinal axis during
shooting
Shooting is the act or process of discharging a projectile from a ranged weapon (such as a gun, bow, crossbow, slingshot, or blowpipe). Even the acts of launching flame, artillery, darts, harpoons, grenades, rockets, and guided missiles ...
to stabilize the projectile longitudinally by
conservation of angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syste ...
, improving its
aerodynamic
Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
stability and accuracy over
smoothbore
A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.
History
Early firearms had smoothly bored barrels that fired projectiles without signi ...
designs.
Rifling is characterized 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), "1 turn in 254
mm" ("1:254 mm" or "1:25.4
cm)", or the like. Normally, an experienced shooter can infer the units of measurement from the numbers alone. A shorter distance indicates a faster twist, meaning that for a given
velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
the projectile will rotate at a higher spin rate.
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.
In some cases, rifling will increase the twist rate as the projectile travels down the length of the barrel, called a ''gain twist'' or ''progressive twist''; a twist rate that decreases from
breech
Breech may refer to:
* Breech (firearms), the opening at the rear of a gun barrel where the cartridge is inserted in a breech-loading weapon
* breech, the lower part of a pulley block
* breech, the penetration of a boiler where exhaust gases leav ...
to
muzzle 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; they are often stabilized aerodynamically instead. An aerodynamically stabilized projectile can be fired from a
smoothbore
A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.
History
Early firearms had smoothly bored barrels that fired projectiles without signi ...
barrel without a reduction in accuracy.
History
Muskets were
smoothbore
A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.
History
Early firearms had smoothly bored barrels that fired projectiles without signi ...
, 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 collect
gunpowder residue.
Some of the earliest recorded European attempts of spiral-grooved musket barrels were of Gaspard Kolner, a gunsmith of
Vienna
en, Viennese
, iso_code = AT-9
, registration_plate = W
, postal_code_type = Postal code
, postal_code =
, timezone = CET
, utc_offset = +1
, timezone_DST ...
in 1498 and Augustus Kotter of
Nuremberg
Nuremberg ( ; german: link=no, Nürnberg ; in the local East Franconian dialect: ''Nämberch'' ) is the second-largest city of the German state of Bavaria after its capital Munich, and its 518,370 (2019) inhabitants make it the 14th-largest ...
in 1520. Some scholars allege that Kollner’s works at the end of the 15th century only used straight grooves, and it wasn’t 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 were
breech loaders such as the
Queen Anne pistol.
Recent developments
Polygonal rifling
![Hammer forged 6-right polygonal rifling pattern](https://upload.wikimedia.org/wikipedia/commons/b/b4/Hammer_forged_6-right_polygonal_rifling_pattern.png)
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 in
handguns. 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 from
CZ,
Heckler & Koch
Heckler & Koch GmbH (HK; ) is a German defense manufacturing company that manufactures handguns, rifles, submachine guns, and grenade launchers. The company is located in Oberndorf am Neckar in the German state of Baden-Württemberg, and also ...
,
Glock
Glock is a brand of polymer-Receiver (firearms), framed, Recoil operation#Short recoil operation, short recoil-operated, locked-breech semi-automatic pistols designed and produced by Austrian manufacturer Glock Ges.m.b.H., Glock Ges.m.b.H. The ...
,
Tanfoglio
Fratelli Tanfoglio S.N.C. is an Italian firearms manufacturing company. Their current weapons are used extensively in sport competitions and for personal defence. Tanfoglio is located in Gardone Val Trompia (Brescia), Italy, and is known for its ...
, and
Kahr Arms, as well as the
Desert Eagle
The Desert Eagle is a gas-operated, semi-automatic pistol known for chambering the .50 Action Express, the largest centerfire cartridge of any magazine-fed, self-loading pistol.
Magnum Research Inc. (MRI) designed and developed the Desert Eagle ...
.
Extended range, full bore
For tanks and artillery pieces, the extended range, full bore (ERFB) concept developed by
Gerald Bull
Gerald Vincent Bull (March 9, 1928 – March 22, 1990) was a Canadian engineer who developed long-range artillery. He moved from project to project in his quest to economically launch a satellite using a huge artillery piece, to which end he des ...
for the
GC-45 howitzer
The GC-45 (''Gun, Canada, 45-calibre'') is a 155 mm howitzer designed by Gerald Bull's Space Research Corporation (SRC) in the 1970s. Versions were produced by a number of companies during the 1980s, notably in Austria and South Africa.
The ...
reverses the normal rifling idea by using a projectile with small nubs that ride in the grooves, instead of having a slightly oversized
driving band Russian 122 mm shrapnel shell, which has been fired, showing rifling marks on the copper driving band around its base and the steel bourrelet nearer the front
A driving band or rotating band is a band of soft metal near the base of an artillery ...
which is forced into the grooves. This imparts spin but eliminates the friction required to deform the driving band into the grooves; a soft and low-friction silicon rubber ring is used to achieve a seal between the projectile and the walls of the tube. Guns capable of firing these projectiles have achieved significant increases in muzzle velocity and range. Unlike a shell narrower than the gun's bore with a
sabot
Sabot may refer to:
* Sabot (firearms), disposable supportive device used in gunpowder ammunitions to fit/patch around a sub-caliber projectile
* Sabot (shoe), a type of wooden shoe
People
* Dick Sabot (1944–2005), American economist and busi ...
, ERFB shells use the full bore, permitting a larger payload. Examples include the South African
G5 and the German
PzH 2000. ERFB may be combined with
base bleed
Base bleed is a system used on some artillery shells to increase range, typically by about 20–35%. It expels gas into the low pressure area behind the shell to reduce base drag (it does not produce thrust).
Since base bleed extends the ran ...
.
Gain-twist rifling
A gain-twist or progressive rifling begins with a slow
twist rate
In firearms, rifling is machining helical grooves 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 to stabilize the pr ...
that gradually increases down the bore, resulting in very little initial change in the projectile's
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
during the first few inches of bullet travel after it enters the
throat
In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebrae. It contains the pharynx and larynx. An important section of it is the epiglottis, separating the esophagus from the trachea (windpip ...
. 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 to
accelerate
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by t ...
d
angular momentum
In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...
as it gets propelled down the barrel. The theoretical advantage is that by gradually increasing the spin rate, torque gets imparted along a much longer bore length, allowing thermomechanical
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
to be spread over a larger area rather than being focused predominantly at the throat, which typically
wear
Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology.
Wear in ...
s out much faster than other parts of the barrel.
Gain-twist rifling was used prior to and during the
American Civil War
The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States. It was fought between the Union ("the North") and the Confederacy ("the South"), the latter formed by states ...
(1861–65). Colt
Army
An army (from Old French ''armee'', itself derived from the Latin verb ''armāre'', meaning "to arm", and related to the Latin noun ''arma'', meaning "arms" or "weapons"), ground force or land force is a fighting force that fights primarily on ...
and
Navy
A navy, naval force, or maritime force is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake-borne, riverine, littoral, or ocean-borne combat operations and related functions. It in ...
revolvers 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 the
M61 Vulcan
The M61 Vulcan is a hydraulically, electrically, or pneumatically driven, six- barrel, air-cooled, electrically fired Gatling-style rotary cannon which fires rounds at an extremely high rate (typically 6,000 rounds per minute). The M61 and i ...
Gatling gun used in some current fighter jets and the larger
GAU-8 Avenger
The General Electric GAU-8/A Avenger is a 30 mm hydraulically driven seven-barrel Gatling-style autocannon that is primarily mounted in the United States Air Force's Fairchild Republic A-10 Thunderbolt II. Designed to destroy a wide variety of g ...
Gatling 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 the
Smith & Wesson Model 460 (X-treme Velocity Revolver).
Manufacture
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 wooden
dowel which were gradually packed out with slips of paper until the required depth was obtained. The process was finished off by casting a
slug of molten lead into the barrel, withdrawing it and using it with a paste of
emery and oil to smooth the bore.
Most rifling is created by either:
*cutting one groove at a time with a
tool
A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates ba ...
(cut rifling or single point cut rifling);
*cutting all grooves in one pass with a special progressive
broaching bit (broached rifling);
*
pressing all grooves at once with a tool called a "button" that is pushed or pulled down the barrel (button rifling);
*
forging the
barrel over a
mandrel containing a reverse image of the rifling, and often the chamber as well (hammer forging);
*
flow forming the
barrel preform over a
mandrel containing a reverse image of the rifling (rifling by flow forming)
*Using non-contact forces such as
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
or
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
from
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
source to
etch the rifling pattern (etching rifling)
*Machine the rifling grooves texture on a thin metal plate, then fold the plate into the inner bore of the barrel (liner rifling)
The ''grooves'' are the spaces that are cut out, and the resulting ridges are called ''lands''. 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 a ''gain 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,
gunsmiths who are
machining a 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.
Construction and operation
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, though it can also take the shape of a
polygon
In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two to ...
, 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 (the diameter across the ''lands'' or high points in the rifling), or by groove diameter (the diameter across the ''grooves'' or low points in the rifling). Differences in naming conventions for
cartridges can cause confusion; for example, the projectiles of the
.303 British
The .303 British (designated as the 303 British by the C.I.P. and SAAMI) or 7.7×56mmR, is a calibre rimmed rifle cartridge. The .303 inch bore diameter is measured between rifling lands as is the common practice in Europe which follows th ...
are actually slightly larger in diameter than the projectiles of the
.308 Winchester
The .308 Winchester is a smokeless powder rimless bottlenecked rifle cartridge widely used for hunting, target shooting, police, military, and personal protection applications globally. It is similar but not identical to the 7.62×51mm NATO ...
, 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 will
swage or
obturate
In the field of firearms and airguns, obturation denotes necessary barrel blockage or fit by a deformed soft projectile (obturation in general is closing up an opening). A bullet or pellet, made of soft material and often with a concave base, ...
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.
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.
Freebore is a groove-diameter length of
smoothbore
A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.
History
Early firearms had smoothly bored barrels that fired projectiles without signi ...
barrel without lands forward of the throat. Freebore allows the bullet to transition from
static friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of t ...
to sliding friction and gain linear
momentum prior 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 of
internal ballistics
Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile.
In guns, internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The s ...
before the bullet starts moving down the barrel. Barrels with freebore length exceeding the rifled length have been known by a variety of trade names including
paradox
A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically u ...
.
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 called ''engraving''. 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 in
forensic ballistics
Ballistics is the field of mechanics concerned with the launching, flight behaviour and impact effects of projectiles, especially ranged weapon munitions such as bullets, unguided bombs, rockets or the like; the science or art of designing an ...
.
Fitting the projectile to the bore
![Shell La Hitte](https://upload.wikimedia.org/wikipedia/commons/2/2c/Shell_La_Hitte.jpg)
The original firearms were
loaded from the muzzle by 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 the ''windage'' (the gap between the ball and the walls of the bore). The patch acted as a
wadding
Wadding is a disc of material used in guns to seal gas behind a projectile (a bullet or ball), or to separate the propellant from loosely packed shots.
Wadding can be crucial to a gun's efficiency, since any gas that leaks past a projectile as i ...
and provided some degree of
pressure sealing, kept the ball seated on the charge of
black 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-based
Minié ball
The Minié ball or Minie ball, is a type of hollow-based bullet designed by Claude-Étienne Minié, inventor of the French Minié rifle, for muzzle-loading rifled muskets. It was invented in 1847 and came to prominence in the Crimean War and ...
, 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.
In
breech-loading firearms
A breechloader is a firearm in which the user loads the ammunition (cartridge or shell) via the rear (breech) end of its barrel, as opposed to a muzzleloader, which loads ammunition via the front ( muzzle).
Modern firearms are generally bre ...
, the task of seating the projectile into the rifling is handled by the ''throat'' of the
chamber
Chamber or the chamber may refer to:
In government and organizations
* Chamber of commerce, an organization of business owners to promote commercial interests
*Legislative chamber, in politics
* Debate chamber, the space or room that houses delib ...
. Next is the ''freebore'', which is the portion of the throat down which the projectile travels before the rifling starts. The last section of the throat is the ''throat 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.
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, 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 in
muzzle-loading
A muzzleloader is any firearm into which the projectile and the propellant charge is loaded from the muzzle of the gun (i.e., from the forward, open end of the gun's barrel). This is distinct from the modern (higher tech and harder to make) desig ...
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
The M16 rifle (officially designated Rifle, Caliber 5.56 mm, M16) is a family of military rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-roun ...
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
An AR-15-style rifle is any lightweight semi-automatic rifle based on the Colt AR-15 design. The original ArmaLite AR-15 is a scaled-down derivative of Eugene Stoner's ArmaLite AR-10 design. The then Fairchild Engine and Airplane Corporatio ...
rifles are commonly found with 1 in or 54.8 calibers for older rifles and 1 in or 41.1 calibers for most newer rifles, although some are made with 1 in 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.
Expressing twist rate
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 is ''relatively'' slow 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.
where:
* Twist = twist rate expressed in bore diameters
* L = the twist length required to complete one full projectile revolution (in mm or in)
* D
bore = bore diameter (diameter of the lands, in mm or in)
Note that the twist travel L and the bore diameter D 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.
Note that the latter two methods have the inherent advantage of expressing twist rate as a ratio and give an easy understanding if a twist rate is ''relatively'' slow or fast even when comparing bores of differing diameters.
Twist rate and bullet stability
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, needing no allowances for weight or nose shape. The eponymous ''Greenhill Formula'', still used today, is:
where:
*C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
*D = bullet's diameter in inches
*L = bullet's length in inches
*SG = bullet's
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
(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 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, 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 projectile
jacket ruptures 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 powder
Finnish smokeless powderSmokeless powder is a type of propellant used in firearms and artillery that produces less smoke and less fouling when fired compared to gunpowder ("black powder"). The combustion products are mainly gaseous, compared t ...
can produce muzzle velocities of approximately for spin stabilized projectiles and more advanced propellants used in
smoothbore
A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.
History
Early firearms had smoothly bored barrels that fired projectiles without signi ...
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
In geometry, coaxial means that several three-dimensional linear or planar forms share a common axis. The two-dimensional analog is ''concentric''.
Common examples:
A coaxial cable is a three-dimensional linear structure. It has a wire condu ...
to the bore, and excess twist will exacerbate the accuracy problems this causes.
Bullet spin
A bullet fired from a rifled barrel can spin at over 300,000
rpm
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
(5
kHz), depending on the bullet's
muzzle velocity and the barrel's
twist rate
In firearms, rifling is machining helical grooves 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 to stabilize the pr ...
.
The general definition of the spin
of an object rotating around a single axis can be written as
:
where
is the linear
velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
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 = 5.2 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.
See also
*
Rifle
*
Smoothbore
A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.
History
Early firearms had smoothly bored barrels that fired projectiles without signi ...
*
Paradox gun
A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically u ...
*
Rifled musket
*
Comparison microscope
A comparison microscope is a device used to analyze side-by-side specimens. It consists of two microscopes connected by an optical bridge, which results in a split view window enabling two separate objects to be viewed simultaneously. This avoids t ...
*
Gun barrel sequence
The gun barrel sequence is a signature device featured in nearly every List of James Bond films, ''James Bond'' film.Cork, John & Scivally, Bruce (2002). ''James Bond: The Legacy''. Boxtree, 46. Shot from the point of view of a presumed assassin, ...
in ''James Bond'' films
*
Greenhill formula
Sir Alfred George Greenhill, FRS FRAeS (29 November 1847 in London – 10 February 1927 in London), was a British mathematician.
George Greenhill was educated at Christ's Hospital School and from there he went to St John's College, Cambridge i ...
*
Glossary of firearms terminology
The following are terms related to firearms and ammunition topics.
A
*Accurize, accurizing: The process of altering a stock firearm to improve its accuracy.
* Action: The physical mechanism that manipulates cartridges and/or seals the breech. ...
*
Miller twist rate
References
External links
Articleon barrel making from an
IHMSA
Metallic silhouette shooting is a group of target shooting disciplines that involves shooting at steel targets representing game animals at varying distances, seeking to knock the metal target over. Metallic silhouette is shot with large bore rif ...
shooter
Articleon barrel making from Lilja, a maker of world class competition barrels
on making and measuring rifling by Lilja; includes pictures of button rifling machine
Article on rifle barrel manufacturing process including gundrilling, reaming, rifling, and finishing
on barrels
explaining now to determine the true bore and groove size and choose appropriate bullet diameters
Calculating Bullet RPM — Spin Rates And StabilityCommon rifling dimensions of revolver, pistol and rifle ammunition
Calculators for stability and twist
Miller Formula CalculatorDrag/Twist Calculator based on Bob McCoy's "McGyro" algorithm
{{Firearms
Artillery components
Firearm components
1498 introductions