High Quality Content by WIKIPEDIA articles! The .338 Winchester Magnum is a .33 caliber (8.38 mm) belted rimless bottlenecked cartridge introduced in 1958 by Winchester Repeating Arms. It is based on the blown out, shortened .375 H&H Magnum. The .33 (8.38mm) is the caliber at which medium bore cartridges are considered to begin. Hence, the .338 Winchester Magnum is considered a first option for those requiring a medium bore cartridge. It is the most popular medium bore cartridge in North America and one of the most popular magnum cartridges currently available. 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.