The 270 Winchester Short Magnum or 270 WSM is a short, unbelted, magnum cartridge created by necking down the .300 Winchester Short Magnum and fitting it with a .277 caliber bullet. The correct name for the cartridge, as listed by the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), is 270 WSM, without a decimal point.
.270 Winchester Short Magnum Wikipedia
When it was introduced, the 300 WSM sported a new case that showed a lot of promise for uses in other calibers. In 2002 Winchester introduced new cartridges in its Winchester Short Magnum family, including the 7 mm WSM and 270 WSM. This new .270 cartridge was the third commercial .270 ever produced, and the first one in 60 years.
The 270 WSM is an improvement over the older 270 Winchester providing higher velocity with bullets of the same weight, and thus a flatter trajectory and more energy. Velocities tend to be about 250 ft/s (76 m/s) faster, in a cartridge that is shorter and can therefore be used in a shorter action for a more compact rifle if desired.
The 270 WSM is the only cartridge of the WSM family that produces notable ballistic gains over other existing cartridges. The .300 WSM closely mimics the long popular .300 Winchester Magnum, while the 7mm WSM lags slightly behind the common 7mm Remington Magnum in terms of performance. The .325 WSM is in a league of its own in North America as the 8 mm caliber has not become commonplace.
The 270 WSM comes quite close in performance to that of the legendary .270 Weatherby Magnum. Except that the WSM is chambered in a short action and usually sports a 22" to 24" barrel where the Weatherby is usually chambered in rifles with a 26" barrel.