Heres an article I found put out by the folks at Kimber-

In 2002, most shooters and hunters expected Winchester to add a larger caliber to the WSM line and the popular guess was 338. However, Winchester surprised the shooting public with the introduction of two Winchester Super Short Magnums, the 223 and 243. Here was a totally new concept in commercially available ammunition. The "short-fat" cartridges were designed to work with new Super Short action rifles from Browning and Winchester Firearms. Then in 2003, a 25 caliber load was added to the WSSM line.

Shooters and hunters who like bigger bore rifles for large game hunting are expecting Winchester to finally make the move to bigger bullets in the WSM line for 2005. They are right, but they may be surprised.

Winchester recognized the need for a cartridge capable of launching 200 plus grain bullets with high inherent accuracy, energy capable of stopping the largest North American game, and lower perceived recoil. Winchester engineers considered several different calibers during development and determined that the 325 caliber was the best performer in the WSM platform. This new cartridge delivers similar energies as a 338 Win-Mag in a much smaller lightweight package.

The platform of the "Short-Fat" cartridge design has for years consistently produced very accurate cartridges. Since their introduction, the Winchester Short Magnum and the Winchester Super Short Magnum calibers have all produced exceptional accuracy. In addition to delivering excellent ballistics, the 325 WSM also exhibits exceptional accuracy, a real plus in this large caliber cartridge.

Initially, three loads will be available in the 325 WSM. Winchester Ammunition and Nosler, Inc. developed a 200 gr. Accubond® CT® in the Supreme® line and in the Super-X® line, Winchester Ammunition developed a 220 gr. Power-Point® bullet. Both of these loads utilize bullets specifically designed for use in the 325 WSM. Additionally, a 180 gr. Ballistic Silvertip® in the Supreme® line will be available. Additional loads are expected in the future.

The 325 WSM is the ideal package for the serious elk, bear, moose or other large and dangerous game hunter where a lightweight short magnum rifle is desired.

For more information on Winchester® products, please visit their website at www.winchester.com . Press releases and images can be found on the website at www.winchester.com , then click on Press Room.