Polygonal rifling

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

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, Noveske Rifleworks and Kahr Arms. Polygonal rifling is usually only found in pistol barrels, and is less common in rifles, However some extremely high end rifles like the PSG-1 use polygonal bores.

The term "polygonal rifling" is fairly general, and different manufacturers employ varying polygonal rifling profiles.

Advantages

A number of advantages are claimed by the supporters of polygonal rifling. These include:
*Higher velocities due to reduced friction of the bullet in the barrel, as the polygonal rifling has less surface area than the lands and grooves of a traditionally rifled barrel
*Less bullet deformation, resulting in reduced drag on the bullet which helps to increase range and accuracy
*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.

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. Noted firearms expert and barrel maker, the late Gale McMillan, has also commented that lead bullets and polygonal rifling are not a good mix. However, since neither H&K nor Kahr recommend against lead bullets in their polygonal rifled barrels, it is probable that there is an additional factor involved in Glock's warning. One explanation 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. The other explanation is that Glock's barrels may be more prone than normal to leading, which 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 can become constricted and result in higher than normal pressures.

ee also

*Ballistic fingerprinting
*Smoothbore

External links

* [http://glockmeister.com/faq.shtml Glockmeister FAQ] , with information on lead bullets in Glock firearms.
* [http://www.thegunzone.com/let-2001.html The Gun Zone 2001 e-mail questions] , with information on cast bullets in Glock and H&K handguns.
* [http://www.lasc.us/RangingShotBarrelMakingFeature.htm Barrel making] FAQ, with information on methods of making and rifling barrels
* [http://www.6mmbr.citymaker.com/barrelFAQ.html 6mmBR barrel FAQ] , covers new polygonal profile button rifled barrels
* [http://yarchive.net/gun/ammo/glock_lead_bullet.html Polygonal Rifling] , A comment from Gale McMillan about lead bullets and polygonal rifling.