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Polygonal rifling is a type of rifling wherein the traditional lands and grooves are replaced instead by "hills and valleys" in a rounded polygonal pattern, usually a hexagon.
History
While polygonal rifling has been around since the earliest days of rifled barrels, Heckler & Koch was the first manufacturer to begin using polygonal rifling in modern arms. Companies that utilize this method today include Heckler & Koch, GLOCK and Kahr arms. Polygonal rifling is usually only found in pistol barrels, and is less common in rifles.
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, it should be noted that 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. The areas where polygonal rifled barrels are used competitively is 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 take a bored barrel blank and rifle, chamber, and contour it on one step. Invented 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 thus 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 lead to a widespread belief that polygonal rifling is not compatable with lead bullets. However, this appears to be due to other properties of the GLOCK barrel, as neither H&K nor Kahr recommend against lead bullets. The 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. Since the GLOCK will fire while slightly out of battery (by design, as this increases reliabilty under adverse conditions), and since shooting lead bullets may result in failures to fully return to battery, shooting lead bullets can lead to an increased risk of a case head failure.
See also
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"