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.

[2] The correct name for the cartridge, as listed by the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), is 243 WSSM, without a decimal point.

[5] The .243 WSSM's case is unusually short and fat in profile, contrasting markedly with most other rifle cartridges, and is intended to take advantage of what ballisticians have shown is the more uniform and efficient burning of propellant powder when it is held in a short, fat stack by the cartridge case.

[6] In their ballistics tables, Winchester list a very high muzzle velocity of 4,060 ft/s (1,240 m/s) with a 55-grain (3.6 g) projectile for this cartridge.