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. 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.