Polygonal rifling

Polygonal riflings with a larger number of edges have shallower corners, which provide a better gas seal in relatively large diameter bores.

[1] The principle of the polygonal barrel was proposed in 1853 by Sir Joseph Whitworth, a prominent British engineer and entrepreneur.

The Whitworth sharpshooters killed multiple high-ranking Union officers, most famously Major General John Sedgwick, who was fatally shot at a range of 1,000 yards (910 m) during the Battle of Spotsylvania Court House.

[3] Companies that utilize this method today include Tanfoglio, Heckler & Koch, Glock (Gen 1-4), Magnum Research, Česká Zbrojovka, Kahr Arms, Walther and Israel Weapon Industries.

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 rifling in Germany in 1939, hammer forging has remained popular in Europe but was only later used by gunmakers in the United States.

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.