The .401 Winchester Self-Loading (also called .401SL or .401WSL) is an American rifle cartridge.
Winchester introduced the .401SL in the Winchester '10 self-loading rifle as a supplement to the Winchester '07 and the .351SL in their offering of hi-power, self-loading rifles. The only chambering available in the Winchester Model 1910, the .401SL was used by France, Russia, and American company security forces in the First World War.
The .401SL proved powerful enough for both deer and other large game at ranges under 150 yards. Both 200gr and 250gr bullet weights were offered by Winchester and other ammunition manufacturers as factory loadings. With extra available detachable magazines holding 4-rounds each, the Model '10, could provide lots of firepower for the big-game hunter. This feature helped promote the use of the .401SL on dangerous game such as moose and grizzly bear in spite of the lack of controlled expansion bullet designs, which doubtlessly would have improved game-taking performance and the subsequent reputation of the .401SL cartridge.
The .401 SL is of similar size to the later .41 Remington Magnum; but the longer self-loading rifle cartridge produced a muzzle energy of 2,000 foot-pounds force (2,700 J) with a 200-grain (13 g) bullet, while the magnum revolver is credited with a muzzle energy of 790 foot-pounds force (1,070 J) with a 210-grain (14 g) bullet. The .41 Rem magnum revolver comparison is not entirely relevant, however, since in a carbine the same cartridge generally at least doubles its energy over the much shorter barreled version.