Polygonal rifling

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.

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