the 243 winchester super short magnum or 243 wssm is a rifle cartridge introduced in 2003. it uses a .300 wsm (winchester short magnum) case shortened and necked down to accept a .243in/6mm diameter bullet, and is a high velocity round based on ballistics design philosophies that are intended to produce a high level of efficiency. the correct name for the cartridge, as listed by the sporting arms and ammunition manufacturers' institute (saami), is 243 wssm, without a decimal point. winchester has discontinued the manufacture of 243 wssm ammunition.