Polygonal rifling (/pəˈlɪɡənəl/ pə-LIG-ə-nəl) is a type of gun barrel rifling where the traditional lands and grooves are replaced by "hills and valleys" in a rounded polygonal pattern, usually taking the form of 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, for instance, octagonal 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 more closely than the hexagonal rifling used in smaller diameter bores.
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 cartridges.
The principle of the polygonal barrel was proposed in 1853 by Sir Joseph Whitworth, a prominent British engineer and entrepreneur. Whitworth experimented with cannons using twisted hexagonal barrels instead of traditional round rifled barrels, which was patented in 1854. In 1856, this concept was demonstrated in a series of experiments using brass howitzers. The British military, however, rejected Whitworth's polygonal rifled designs. Whitworth believed that polygonal rifling could be used to create a more accurate rifled musket to replace the Pattern 1853 Enfield. During the American Civil War, Whitworth's polygonal rifled Whitworth rifle was successfully used by the Confederate States Army Whitworth Sharpshooters. The Whitworth rifle is often called the 'sharpshooter' because of its accuracy compared to other rifled muskets of its era (far surpassing the Sharps), and is considered one of the earliest examples of a sniper rifle. The last common rifles to use polygonal rifling were the Lee–Metford rifle, named after the Metford rifling, and the Arisaka rifles designed by Col. Arisaka. The switch to cordite 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 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 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 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.
Companies that utilize this method today include Tanfoglio, Heckler & Koch, Glock, Magnum Research, Česká Zbrojovka, Kahr Arms and Walther. Polygonal rifling is usually found only in pistol barrels, and is less common in rifles; however, some extremely high end rifles like the Heckler & Koch PSG1, Precision Sniper Rifle PSR 90, and the 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 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.
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.
Less sensitive to stress concentration induced barrel failure.
Prolonged barrel life.
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 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 in 1939, hammer forging has remained popular in Europe, but was only later used by gunmakers in the United States. The hammer forging process produces large amounts of stress in the barrel that must be relieved by careful heat treatment, a process that is less necessary in a traditionally cut or button rifled barrel. Due to the potential for residual stress 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.
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. However, it must be noted that 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.
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. Neither H&K nor Kahr explicitly recommend against lead bullets in their polygonal rifled barrels, suggesting that there might be an additional factor involved in Glock's warning. Kahr's warns that lead bullets can cause additional fouling and recommends special attention to cleaning after their use. While H&K doesn't insist on a copper jacket, 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, 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.