Introduced in 2003, the 243 WSSM (Winchester Super Short Magnum) was a joint engineering/marketing plan by Winchester/Olin (Ammuniton), U.S. Repeating Arms Company (Winchester rifles and shotguns licensee), and the Browning Company. The cartridge is shorter and fatter than its counterparts and therefore performs with greater speed and better ballistics. The increased speed is achieved by putting a lightweight bullet on top of a short, fat, high-volume case. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.