Polygonal rifling
Polygonal rifling is a type of gun barrel rifling where the traditional sharp-edged "lands and grooves" are replaced by less pronounced "hills and valleys", so the barrel bore has a polygonal cross-sectional profile.
Polygonal riflings with a larger number of edges have shallower corners, which provide a better gas seal in relatively large diameter bores. For instance, in the pre-Gen 5 Glock pistols, octagonal rifling is used in the large diameter.45 ACP bore, which has an 11.23 mm diameter, since it resembles a circle more closely than the hexagonal rifling used in smaller diameter bores.
History
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 and patented the design in 1854. In 1856, this concept was demonstrated in a series of experiments using brass howitzers. The British military, however, rejected Whitworth's polygonal designs. Afterwards, Whitworth adopted the concept on small arms, believing 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 polygonally rifled Whitworth rifle was successfully used by the Confederate States Army marksmen to terrorize Union Army artillery crews. The muzzle-loading Whitworth rifle is often called the "sharpshooter" because of its superior accuracy compared to other rifled muskets of its era and is considered one of the earliest examples of a sniper rifle. The Whitworth sharpshooters killed multiple high-ranking Union officers, most famously Major General John Sedgwick, who was fatally shot at a range of during the Battle of Spotsylvania Court House.
The last service rifles to use polygonal rifling were the British Lee–Metford rifles, named after their proprietary Metford rifling, American M1895 Lee Navy rifles, and the Japanese Arisaka rifles designed by Colonel Arisaka and Colonel Nambu. The Lee-Metford rifle turned out to be a failure after the switch to the erosive cordite proved too much for the smooth and shallow Metford rifling, which had been designed to reduce barrel fouling for black powder ammunition. When the Metford rifling design was dropped, the Lee–Metford became the Lee–Enfield rifle in favour of Enfield type rifling, which was deep-grooved for longer service life. Lee Navy shared the same fate since Rifleite adopted for 6 mm Lee Navy was analogous to cordite. However, the Arisaka rifles were manufactured extensively for the Imperial Japanese Army from 1897 to 1945 with no excessive rifling erosion problems, as the Japanese had adopted a better, non-erosive, rifle powder.
During World War II, polygonal rifling emerged again in the German MG 42 general-purpose machine guns, as an outgrowth of a cold-hammer forging process developed by German engineers prior to the outbreak of the war. The process addressed the need to produce large quantities of more durable gun barrels in less time than those produced with traditional methods, as the MG 42's infamously fast rate of fire tended to overheat the barrel quickly and thus warranted frequent barrel changes. The MG 42's 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 battle 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, Walther and Israel Weapon Industries. Polygonal rifling is usually found only in pistol barrels, and is less common in rifles; however, some extremely high end precision rifles like the Heckler & Koch PSG1 and its Pakistani variant PSR-90, and the LaRue Tactical Stealth System sniper rifle use polygonal bores.
Design
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, and also less sensitive to stress concentration-induced barrel failure.
- Providing a better gas seal around the projectile as polygonal bores tend to have shallower, smoother edges with a slightly smaller bore area, which translates into more efficient seal of the combustion gases trapped behind the bullet, slightly greater muzzle velocities and slightly increased accuracy.
- Less bullet deformation, resulting in less frictional resistance when the bullet travels through the barrel, which helps to increase muzzle velocity. The lack of sharp surface deformation on the bullet also reduces drag in flight.
- Reduced buildup of copper or lead within the barrel, as there are no sharp rifling edges to "shred" into the bullet surface and no pronounced corners that can accumulate foulings difficult to clean, which results in easier maintenance. The reduced fouling also theoretically translates to a simpler "copper equilibrium" profile, which is potentially beneficial to accuracy.
- Prolonged barrel life, as the thermomechanical stress upon the riflings are spread over a larger area, hence less wear over time.
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, 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 in the United States 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.