Polygonal rifling

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Conventional eight groove rifling on the left, and octagonal polygonal rifling on the right.

Polygonal rifling is a type of rifling wherein the traditional lands and grooves are replaced by "hills and valleys" in a rounded polygonal pattern, usually a hexagon or octagon.

History

Left image: Rifled mountain cannon of the French La Hitte system, "Canon de montagne de 4 modèle 1859 Le Pétulant". Caliber: 86 mm. Length: 0.82 m. Weight: 101 kg (208 kg with carriage). Ammunition: 4 kg shell.
Right image: Hexagonal rifling of Le Pétulant (detail).

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 last common rifle to use polygonal rifling was the Lee-Metford rifle, named after the Metford rifling, a seven sided polygonal type rifling. The switch to cordite from black powder proved too much for the shallow rifling in the relatively soft barrels of the time, and the Lee-Metford became the Lee-Enfield when the Metford rifling was dropped. Heckler & Koch was the first manufacturer to begin using polygonal rifling in modern arms. Companies that utilize this method today include Heckler & Koch, Glock, Magnum Research, Česká Zbrojovka and Kahr Arms. 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 PSG-1 or the Mexican FX-05 rifles 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:

• Less bullet deformation, resulting in reduced drag on the bullet which helps to increase range, accuracy, and velocity
• Increased barrel life and reduced buildup of copper or lead within the barrel

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 gun barrel 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 service pistol or a typical hunting rifle.

Lead bullets and polygonal rifling

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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]^[1] 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^[2] appears to 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.

See also

• Ballistic fingerprinting
• Smoothbore

References

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.