.338 Winchester Magnum is a .338 in (8.6 mm) caliber, belted, rimless, bottlenecked cartridge introduced in 1958 by Winchester Repeating Arms. It is based on the blown-out, shortened .375 H&H Magnum. The .338 Winchester Magnum is the caliber at which medium-bore cartridges are considered to begin. The .338 Winchester Magnum is the first choice among professional brown bear (specifically grizzly bear) guides in Alaska to back up clients where a powerful stopping caliber is required on charging bears. It is also the most popular medium-bore cartridge in North America and has the most widely available choice in rifles among medium bore rifles. The action length is the same as a .30-06 and most major rifle manufacturers in the United States chamber rifles for the cartridge including the semi-automatic Browning BAR Mk II Safari, making it a very powerful combination against charging dangerous game. The cartridge was intended for larger North American big-game species and has found use as for the hunting of thin-skinned African plains-game species.

The .338 Winchester Magnum traces its heritage to the experiments conducted by Charles O’Neil, Elmer Keith and Don Hopkins with cartridges firing .333 in (8.5 mm) bullets in the late 1940s. The use of .333 in (8.5 mm) bullets may seem odd today but at the time this was the standard diameter of European .33 caliber bullets which were more common than the .338 in (8.6 mm) diameter bullets used in cartridges such as the .33 Winchester. Furthermore, the .333 in (8.5 mm) were available in heavier weights than the .338 in (8.6 mm) bullets. O’Neil, Keith and Hopkins experiments led to the creation of the .333 OKH, which was based on the .30-06 Springfield case necked up to accept .33 caliber bullets and the .334 OKH which used a shortened .375 H&H Magnum necked down to accept the same bullets.

The .338 Winchester Magnum was introduced in 1958 together with the .264 Winchester Magnum and the .458 Winchester Magnum, all of which used a common case design based on the .375 H&H Magnum case blown out and shortened to 2.500 in (63.5 mm), much like the .334 OKH. When the cartridge was introduced, Winchester offered a 200 gr (13 g) at 3,000 ft/s (910 m/s), a 250 gr (16 g) at 2,700 ft/s (820 m/s) and a 300 gr (19 g) at 2,450 ft/s (750 m/s). Sometime later, Winchester introduced the Winchester Model 70 Alaskan chambered for the cartridge. This chambering left little doubt that the cartridge was intended for big heavy dangerous game.

Design and specifications
The .338 Winchester Magnum follows the modern cartridge designs in that the cartridge case features a case with minimal taper so as to maximize case capacity yet providing reliable feeding and extraction. The .338 Winchester Magnum is based on the .375 H&H Magnum, which was shortened to 2.500 in (63.5 mm), blown out and necked down to hold a .338 in (8.6 mm) bullet. The case has a capacity of 86.0 grain of H2O (5.58 cm3). The cartridge follows the design of the standard length Weatherby Magnum cartridges such as the .257 Weatherby Magnum, .270 Weatherby Magnum and the 7 mm Weatherby Magnum for which Winchester had supplied basic brass until 1948.

Both the Commission Internationale Permanente pour l’Epreuve des Armes à Feu Portatives(CIP) and the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) govern and regulate the specifications regarding the .338 Winchester Magnum