Polygonal rifling is a type of gun barrel

Gun barrel

A gun barrel is the tube, usually metal, through which a controlled explosion or rapid expansion of gases are released in order to propel a projectile out of the end at a high velocity....

 rifling

Rifling

Rifling is the process of making helical grooves in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis...

 where the traditional lands and grooves are replaced by "hills and valleys" in a rounded polygon

Polygon

In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...

al pattern, usually a hexagon or octagon.

Polygons with a larger number of edges provide a better gas seal in relatively large diameter polygonally rifled bores. In the Glock pistol

Glock pistol

The Glock pistol, sometimes referred to by the manufacturer as Glock "Safe Action" Pistol, is a series of semi-automatic pistols designed and produced by Glock Ges.m.b.H., located in Deutsch-Wagram, Austria. The company's founder, engineer Gaston Glock, had no experience with firearm design or...

, for instance, octagonal rifling is used in the large diameter .45 ACP

.45 ACP

The .45 ACP , also known as the .45 Auto by C.I.P., is a cartridge designed by John Browning in 1904, for use in his prototype Colt semi-automatic .45 pistol and eventually the M1911 pistol adopted by the United States Army in 1911.-Design and history:The U.S...

 bore, which has a 11.23 mm (0.442 in) diameter, since it resembles a circle

Circle

A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

 more closely than the hexagonal rifling used in smaller diameter bores.

History

While polygonal rifling has been around since the earliest days of rifled barrels, it had faded out of use by the time of the early cordite

Cordite

Cordite is a family of smokeless propellants developed and produced in the United Kingdom from 1889 to replace gunpowder as a military propellant. Like gunpowder, cordite is classified as a low explosive because of its slow burning rates and consequently low brisance...

 cartridges. The last common rifles to use polygonal rifling was the Lee-Metford

Lee-Metford

The Lee-Metford rifle was a bolt action British army service rifle, combining James Paris Lee's rear-locking bolt system and ten-round magazine with a seven groove rifled barrel designed by William Ellis Metford...

 rifle, named after the Metford rifling, and the Arisaka rifles designed by Col. Arisaka. The switch to cordite

Cordite

Cordite is a family of smokeless propellants developed and produced in the United Kingdom from 1889 to replace gunpowder as a military propellant. Like gunpowder, cordite is classified as a low explosive because of its slow burning rates and consequently low brisance...

 from black powder proved too much for the shallow rifling in the relatively soft barrels of the Metford, and the Lee-Metford became the Lee-Enfield

Lee-Enfield

The Lee-Enfield bolt-action, magazine-fed, repeating rifle was the main firearm used by the military forces of the British Empire and Commonwealth during the first half of the 20th century...

 when the Metford rifling was dropped, however, Arisakas were manufactured until the surrender of the Imperial Japanese Army in 1945, it has been proven that Arisaka rifling was more durable, and the accuracy of many captured rifles can attest to this.

During World War 2 polygonal rifling emerged again in the German MG 42 machine gun. It was an outgrowth of a cold-hammer forging

Work hardening

Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements within the crystal structure of the material. Any material with a reasonably high melting point such as metals and...

 process developed by German engineers prior to World War 2. The process addressed the need to produce more durable machine gun barrels in less time than those produced with traditional methods. The MG42 successor the Rheinmetall MG 3 machine gun can also have polygonal rifling.
Heckler & Koch

Heckler & Koch

Heckler & Koch GmbH is a German defense manufacturing company that produces various small arms. Some of their products include the SA80, MP5 submachine gun, G3 automatic rifle, the G36 assault rifle, the HK 416, the MP7 personal defense weapon, the USP series of handguns, and the high-precision...

 was the first manufacturer to begin using polygonal rifling in modern small arms like the G3A3 assault rifle and several semi automatic hunting rifles like the HK SL7

HK SL7

The H&K SL7 is a roller-delayed blowback operated sporting carbine made by Heckler & Koch. It is manufactured in 7.62×51mm caliber. It was designed and marketed throughout the world as a hunting/utility rifle....

.

Companies that utilize this method today include Heckler & Koch

Heckler & Koch

Heckler & Koch GmbH is a German defense manufacturing company that produces various small arms. Some of their products include the SA80, MP5 submachine gun, G3 automatic rifle, the G36 assault rifle, the HK 416, the MP7 personal defense weapon, the USP series of handguns, and the high-precision...

, Glock

Glock

Glock Ges.m.b.H. is a weapons manufacturer headquartered in Deutsch-Wagram, Austria, named after its founder, Gaston Glock...

, Magnum Research

Magnum Research

Magnum Research Inc. is a privately held corporation based in Fridley, Minnesota which manufactures and distributes firearms. The majority owners, Jim Skildum and John Risdall , have been with the company since its founding in 1979...

, Česká Zbrojovka

Česká Zbrojovka

Česká Zbrojovka may refer to one of the following Czech firearms makers:* Česká Zbrojovka Strakonice* Česká Zbrojovka Uherský Brod* Česká Zbrojovka firearms...

  and Kahr Arms

Kahr Arms

Kahr Arms is an American small arms manufacturer founded by Kook Jin "Justin" Moon , who currently serves as CEO and President. It is owned by the Saeilo Corporation , a subsidiary of Tongil Group, a South Korean business group associated with the Unification Church...

. Polygonal rifling is usually found only in pistol

Pistol

When distinguished as a subset of handguns, a pistol is a handgun with a chamber that is integral with the barrel, as opposed to a revolver, wherein the chamber is separate from the barrel as a revolving cylinder. Typically, pistols have an effective range of about 100 feet.-History:The pistol...

 barrels, and is less common in rifle

Rifle

A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...

s, however some extremely high end rifles like the Heckler & Koch PSG1

Heckler & Koch PSG1

The PSG1 is a semi-automatic rifle designed by the German company Heckler & Koch of Oberndorf am Neckar.-Development:...

 and LaRue Tactical Stealth System sniper rifle use polygonal bores.

The term "polygonal rifling" is fairly general, and different manufacturers employ varying polygonal rifling profiles.
H&K, CZ and Glock use a female type of polygonal rifling similar to the bore shown above right. 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 (Pac Nor) and LWRC use a rifling more like the conventional rifling with both of the lands sides being sloped but has a flat top and defined corners, this type of rifling is more of a canted land type of rifling than polygonal rifling.

Advantages

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.
• Providing a better gas seal around the projectile as polygonal bores tend to have a slightly smaller bore area, which translates into more efficient use of the combustion gases trapped behind the bullet, slightly greater (consistency in) muzzle velocities and slightly increased accuracy.
• Less bullet deformation, resulting in reduced drag on the bullet when traveling through the barrel which helps to increase muzzle velocity.
• Reduced buildup of copper or lead within the barrel which results in easier maintenance characteristics.
• Prolonged barrel life.

However, precision target pistols such as those used in Bullseye

Bullseye (shooting competition)

Bullseye, also known as conventional pistol, is a shooting sport in which participants shoot handguns at paper targets at fixed distances and time limits. A number of organizations, including the NRA and Civilian Marksmanship Program in the United States, have established rules and keep records for...

 and IHMSA

Metallic silhouette

Metallic silhouette shooting is a group of target shooting disciplines that involves shooting at metal cutouts representing game animals at varying distances. Metallic silhouette shooting can be done with airguns, black powder firearms, modern handguns, or modern rifles...

 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

Action shooting

Action shooting is a term that encompasses target shooting events where the shooter's score is based on both the accuracy and elapsed time. Disciplines covered by the term include:*United States Practical Shooting Association...

, such as IDPA and IPSC

IPSC

IPSC may refer to:* Induced pluripotent stem cell, a type of pluripotent stem cell* International Practical Shooting Confederation, a shooting sport based on the concept of practical shooting...

 competitions.

Part of the difference may be that most polygonal rifling is produced by hammer forging

Forge

A forge is a hearth used for forging. The term "forge" can also refer to the workplace of a smith or a blacksmith, although the term smithy is then more commonly used.The basic smithy contains a forge, also known as a hearth, for heating metals...

 the barrel around a mandrel 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

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 in 1939, hammer forging has remained popular in Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, but was only later used by gunmakers in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. The hammer forging process produces large amounts of stress in the barrel that must be relieved by careful heat treatment

Heat treatment

Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...

, a process that is less necessary in a traditionally cut or button rifled barrel. Due to the potential for residual stress

Residual stress

Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...

 causing accuracy problems, precision shooters 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.

Forensic firearms examination

Polygonal rifling prevents the forensic firearms examiner from microscopically measuring the width of land and groove impressions because the lands and grooves have a rounded profile instead of a well-defined rectangular profile. In the FBI GRC file, the land and groove widths for these firearms are listed as 0.000.

Lead bullets and polygonal rifling

The manufacturer Glock advises against using lead bullets (meaning bullets not covered by a copper jacket) in their polygonally rifled barrels, which has led to a widespread belief that polygonal rifling is not compatible with lead bullets. Firearms expert and barrel maker, the late Gale McMillan, has also commented that lead bullets and polygonal rifling are not a good mix. Some have made a point of the fact that neither H&K nor Kahr explicitly recommend against lead bullets in their polygonal rifled barrels, and feel that it is probable that there is an additional factor involved in Glock's warning. However, Kahr's FAQ does include a warning that lead bullets can cause additional fouling

Fouling

Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance...

 and recommends special attention to cleaning after using them. In addition, while H&K doesn't warn against the use of lead, at least one well-documented catastrophic incident in an H&K pistol may be related to this issue. Furthermore, Dave Spaulding, well-known gun writer, reported in the February/March 2008 issue of Handguns Magazine that when he queried H&K about their polygonally rifled barrels that they commented: "It has been their experience that polygonal rifling will foul with lead at a greater rate than will conventional rifling."

One suggestion of what the "additional factor involved in Glock's warning" might be is that Glock barrels have a fairly sharp transition between the chamber and the rifling, and this area is prone to lead buildup if lead bullets are used. This buildup may result in failures to fully return to battery

Out-of-battery

Out-of-battery refers to the status of a weapon before the action has returned to the normal firing position. The term originates from artillery, referring to a gun that fires before it has been pulled back into its firing position in a gun battery....

, allowing the gun to fire with the case not fully supported by the chamber, leading to a potentially dangerous case failure. However, since this sharp transition is found on most autopistols this speculation is of limited value. The sharp transition or "lip" at the front of the chamber is required to "headspace" the cartridge in most autopistols.

Another possible explanation is that there are different "species" of polygonal rifle and perhaps Glock's peculiar style of polygonal rifling may be more prone to leading than the particular styles employed in the H&K and Kahr barrels.

Leading is the buildup of lead in the bore that happens in nearly all firearms firing high velocity lead bullets. This lead buildup must be cleaned out regularly, or the barrel will gradually become constricted resulting in higher than normal discharge pressures. In the extreme case, increased discharge pressures can result in a catastrophic incident.

External links

• Glockmeister FAQ, with information on lead bullets in Glock firearms.
• The Gun Zone 2001 e-mail questions, with information on cast bullets in Glock and H&K handguns.
• Barrel making FAQ, with information on methods of making and rifling barrels
• 6mmBR barrel FAQ, covers new polygonal profile button rifled barrels
• Polygonal Rifling, A comment from Gale McMillan about lead bullets and polygonal rifling.