Polygonal Rifling
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Polygonal rifling ( ) is a type of
gun barrel A gun barrel is a crucial part of gun-type weapons such as small arms, small firearms, artillery pieces, and air guns. It is the straight shooting tube, usually made of rigid high-strength metal, through which a contained rapid expansion of high ...
rifling Rifling is the term for helical grooves machined into the internal surface of a firearms's barrel for imparting a spin to a projectile to improve its aerodynamic stability and accuracy. It is also the term (as a verb) for creating such groov ...
where the traditional sharp-edged "lands and grooves" are replaced by less pronounced "hills and valleys", so the barrel bore has a
polygon In geometry, a polygon () is a plane figure made up of line segments connected to form a closed polygonal chain. The segments of a closed polygonal chain are called its '' edges'' or ''sides''. The points where two edges meet are the polygon ...
al (usually
hexagon In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is de ...
al or
octagon In geometry, an octagon () is an eight-sided polygon or 8-gon. A '' regular octagon'' has Schläfli symbol and can also be constructed as a quasiregular truncated square, t, which alternates two types of edges. A truncated octagon, t is a ...
al) cross-sectional profile. Polygonal riflings with a larger number of edges have shallower corners, which provide a better gas seal in relatively large diameter bores. For instance, in the pre-Gen 5 Glock pistols,
octagon In geometry, an octagon () is an eight-sided polygon or 8-gon. A '' regular octagon'' has Schläfli symbol and can also be constructed as a quasiregular truncated square, t, which alternates two types of edges. A truncated octagon, t is a ...
al rifling is used in the large diameter .45 ACP bore, which has an 11.23 mm (0.442 in) diameter, since it resembles a
circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre. The distance between any point of the circle and the centre is cal ...
more closely than the
hexagon In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is de ...
al rifling used in smaller diameter bores.Barrel History


History

The principle of the polygonal barrel was proposed in 1853 by Sir
Joseph Whitworth Sir Joseph Whitworth, 1st Baronet (21 December 1803 – 22 January 1887) was an English engineer, entrepreneur, inventor and philanthropist. In 1841, he devised the British Standard Whitworth system, which created an accepted standard for screw ...
, a prominent British engineer and entrepreneur. Whitworth experimented with cannons using twisted hexagonal barrels instead of traditional round rifled barrels and patented the design in 1854. In 1856, this concept was demonstrated in a series of experiments using brass
howitzer The howitzer () is an artillery weapon that falls between a cannon (or field gun) and a mortar. It is capable of both low angle fire like a field gun and high angle fire like a mortar, given the distinction between low and high angle fire break ...
s. The British military, however, rejected Whitworth's polygonal designs. Afterwards, Whitworth adopted the concept on small arms, believing that polygonal rifling could be used to create a more accurate
rifled musket A rifled musket, rifle musket, or rifle-musket is a type of firearm made in the mid-19th century. Originally the term referred only to muskets that had been produced as a smoothbore weapon and later had their Gun barrel, barrels replaced with Ri ...
to replace the Pattern 1853 Enfield. During the
American Civil War The American Civil War (April 12, 1861May 26, 1865; also known by Names of the American Civil War, other names) was a civil war in the United States between the Union (American Civil War), Union ("the North") and the Confederate States of A ...
, Whitworth's polygonally rifled Whitworth rifle was successfully used by the
Confederate States Army The Confederate States Army (CSA), also called the Confederate army or the Southern army, was the Military forces of the Confederate States, military land force of the Confederate States of America (commonly referred to as the Confederacy) duri ...
marksmen (known as the Whitworth Sharpshooters) to terrorize Union Army artillery crews. The
muzzle-loading A muzzleloader is any firearm in which the user loads the projectile and the propellant charge into the muzzle end of the gun (i.e., from the forward, open end of the gun's barrel). This is distinct from the modern designs of breech-loading fire ...
Whitworth rifle is often called the "sharpshooter" because of its superior accuracy compared to other rifled muskets of its era (far surpassing the breechloading Sharps rifle used by the Union Army) and is considered one of the earliest examples of a
sniper rifle A sniper rifle is a high-precision, long range shooting, long-range rifle. Requirements include high accuracy, reliability, mobility, concealment, and optics, for anti-personnel weapon, anti-personnel, anti-materiel rifle, anti-materiel and sur ...
. The Whitworth sharpshooters killed multiple high-ranking Union officers, most famously Major General John Sedgwick, who was fatally shot at a range of during the Battle of Spotsylvania Court House. The last service rifles to use polygonal rifling were the British Lee–Metford rifles, named after their proprietary Metford rifling, American M1895 Lee Navy rifles (both designed by James Paris Lee), and the Japanese Arisaka rifles designed by Colonel Arisaka and Colonel Nambu. The Lee-Metford rifle turned out to be a failure after the switch to the erosive
Cordite Cordite is a family of smokeless propellants developed and produced in Britain since 1889 to replace black powder as a military firearm propellant. Like modern gunpowder, cordite is classified as a low explosive because of its slow burni ...
proved too much for the smooth and shallow Metford rifling, which had been designed to reduce barrel
fouling Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling, organic) or a non-living substance (inorganic). Fouling is usually distinguished from other surfac ...
for
black powder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, charcoal (which is mostly carbon), and potassium nitrate, potassium ni ...
ammunition. When the Metford rifling design was dropped, the Lee–Metford became the
Lee–Enfield The Lee–Enfield is a bolt-action, magazine-fed repeating rifle that served as the main firearm of the military forces of the British Empire and Commonwealth during the first half of the 20th century, and was the standard service rifle of th ...
rifle in favour of Enfield type rifling, which was deep-grooved for longer service life. Lee Navy shared the same fate since ''Rifleite'' adopted for 6 mm Lee Navy was analogous to Cordite. However, the Arisakas were manufactured extensively for the
Imperial Japanese Army The Imperial Japanese Army (IJA; , ''Dai-Nippon Teikoku Rikugun'', "Army of the Greater Japanese Empire") was the principal ground force of the Empire of Japan from 1871 to 1945. It played a central role in Japan’s rapid modernization during th ...
from 1897 to 1945 with no excessive rifling erosion problems, as the Japanese had adopted a better, non-erosive, rifle powder. During
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
, polygonal rifling emerged again in the German MG 42
general-purpose machine gun A general-purpose machine gun (GPMG) is an air-cooled, usually belt-fed machine gun that can be adapted flexibly to various tactical roles for light and medium machine guns. A GPMG typically features a quick-change barrel design calibered fo ...
s, as an outgrowth of a cold-hammer forging process developed by German engineers prior to the outbreak of the war. The process addressed the need to produce large quantities of more durable gun barrels in less time than those produced with traditional methods, as the MG 42's infamously fast rate of fire tended to overheat the barrel quickly and thus warranted frequent barrel changes. The MG 42's successor, the Rheinmetall MG 3 machine gun, can also have polygonal rifling. Heckler & Koch was the first manufacturer to begin using polygonal rifling in modern small arms like the G3A3
battle rifle A battle rifle is a service rifle chambered to fire a fully powered cartridge. The term "battle rifle" is a retronym created largely out of a need to differentiate automatic rifles chambered for fully powered cartridges from automatic rifles cha ...
and several semi-automatic hunting rifles like the HK SL7. Companies that utilize this method today include Tanfoglio, Heckler & Koch,
Glock Glock (; stylized as GLOCK) is a brand of polymer- framed, short-recoil-operated, striker-fired, locked-breech semi-automatic pistols designed and produced by Austrian manufacturer Glock Ges.m.b.H. The firearm entered Austrian military an ...
(Gen 1-4), Magnum Research, Česká Zbrojovka, Kahr Arms, Walther and Israel Weapon Industries. Polygonal rifling is usually found only in
pistol A pistol is a type of handgun, characterised by a gun barrel, barrel with an integral chamber (firearms), chamber. The word "pistol" derives from the Middle French ''pistolet'' (), meaning a small gun or knife, and first appeared in the Englis ...
barrels, and is less common in
rifle A rifle is a long gun, long-barreled firearm designed for accurate shooting and higher stopping power, with a gun barrel, barrel that has a helical or spiralling pattern of grooves (rifling) cut into the bore wall. In keeping with their focus o ...
s; however, some extremely
high end In economics, a luxury good (or upmarket good) is a good (economics), good for which demand (economics), demand increases more than what is proportional as income rises, so that expenditures on the good become a more significant proportion of ove ...
precision rifles like the
Heckler & Koch PSG1 The Heckler & Koch PSG1 (''Präzisionsschützengewehr'', German for "precision marksman rifle") is a semi-automatic designated marksman rifle designed and produced by the German company Heckler & Koch. Development This rifle is said to have ...
and its Pakistani variant PSR-90, and the LaRue Tactical Stealth System sniper rifle use polygonal bores.


Design

A number of advantages are claimed by the supporters of polygonal rifling. These include: *Not compromising the barrel's thickness in the area of each groove as with traditional rifling, and also less sensitive to stress concentration-induced barrel failure. *Providing a better gas seal around the projectile as polygonal bores tend to have shallower, smoother edges with a slightly smaller bore area, which translates into more efficient seal of the combustion gases trapped behind the bullet,Peter Alan Kasler - Glock: The New Wave in Combat Handguns, p. 138 slightly greater (consistency in) muzzle velocities and slightly increased accuracy. *Less bullet deformation, resulting in less frictional resistance when the bullet travels through the barrel, which helps to increase muzzle velocity. The lack of sharp surface deformation on the bullet (rifling marks) also reduces drag in flight. *Reduced buildup of copper or lead within the barrel, as there are no sharp rifling edges to "shred" into the bullet surface and no pronounced corners that can accumulate
fouling Fouling is the accumulation of unwanted material on solid surfaces. The fouling materials can consist of either living organisms (biofouling, organic) or a non-living substance (inorganic). Fouling is usually distinguished from other surfac ...
s difficult to clean, which results in easier
maintenance The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure and supporting utilities in industrial, business, and residential installa ...
. The reduced fouling also theoretically translates to a simpler "copper equilibrium" profile, which is potentially beneficial to accuracy. *Prolonged barrel life, as the thermomechanical stress upon the riflings are spread over a larger area, hence less
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 ...
over time. However, precision target pistols such as those used in Bullseye and IHMSA almost universally use traditional rifling, as do target rifles. The debate among target shooters is almost always one of cut vs. button rifled barrels, as traditional rifling is dominant. Polygonal rifled barrels are used competitively in pistol action shooting, such as IDPA and IPSC competitions. Part of the difference may be that most polygonal rifling is produced by hammer
forging Forging is a manufacturing process involving the shaping of metal using localized compression (physics), compressive forces. The blows are delivered with a hammer (often a power hammer) or a die (manufacturing), die. Forging is often classif ...
the barrel around a
mandrel A mandrel, mandril, or arbor is a tapered tool against which material can be forged, pressed, stretched or shaped (e.g., a ring mandrel - also called a triblet - used by jewellers to increase the diameter of a wedding ring), or a flanged or t ...
containing a reverse impression of the rifling. Hammer forging machines are tremendously expensive, far out of the reach of custom gunsmiths (unless they buy pre-rifled blanks), and so are generally only used for production barrels by large companies. The main advantage of a hammer forging process is that it can rifle, chamber, and contour a bored barrel blank in one step. First applied to rifling in
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
in 1939, hammer forging has remained popular in
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
but was only later used by gunmakers in the
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
. The hammer forging process produces large amounts of stress in the barrel that must be relieved by careful
heat treatment Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are a ...
, a process that is less necessary in a traditionally cut or button rifled barrel. Due to the potential for
residual stress In materials science and solid mechanics, residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, laser peening im ...
causing accuracy problems, precision shooters in the United States tend to avoid hammer forged barrels, and this limits them in the type of available rifling. From a practical standpoint, any accuracy issues resulting from the residual stresses of hammer forging are extremely unlikely to be an issue in a defense or service pistol, or a typical hunting rifle.


Variations

Different manufacturers employ varying polygonal rifling profiles. H&K, CZ and Glock use a female type of polygonal rifling. This type has a smaller bore area than the male type of polygonal rifling designed and used by Lothar Walther. Other companies such as Noveske Rifleworks (Pac Nor) and LWRC use a rifling more like the conventional rifling, with both of each land's sides being sloped but having a flat top and defined corners; this type of rifling is more a canted land type of rifling than polygonal rifling.


Forensic examination

Polygonal rifling prevents the forensic firearms examiner from microscopically measuring the width of land and groove impressions (so-called " ballistic fingerprinting") because the polygonal riflings have a rounded profile instead of well-defined rectangular edges, which causes few noticeable surface deformations. In the
FBI The Federal Bureau of Investigation (FBI) is the domestic Intelligence agency, intelligence and Security agency, security service of the United States and Federal law enforcement in the United States, its principal federal law enforcement ag ...
GRC file, the land and groove widths for these firearms are listed as 0.000. However, forensic identification of firearms (in court-cases, etc.) is based on microscopic examination of tooling marks on the surface of the bore, produced by the manufacturing process and modified by the drag of bullet jackets on that same surface. Thus, the bore surface of individual firearms is always unique.


See also

* Smoothbore


References

{{refs


External links


Glockmeister FAQ
with information on lead bullets in Glock firearms.

with information on cast bullets in Glock and H&K handguns.

FAQ, with information on methods of making and rifling barrels

covers new polygonal profile button rifled barrels

A comment from Gale McMillan about lead bullets and polygonal rifling. Firearm components