The .256 Winchester Magnum is nothing more exotic than the .357 Magnum case necked down for .257" bullets. When the ballistics of the .256 Winchester Magnum are compared with those of other varmint cartridges, its performance in the field is not something one would sneeze at. A 75 grain spitzer moving out at 2500 fps delivers over 600 foot-pounds of residual energy at 200 yards, or about the same as the .222 Remington at the same ranges. Zeroed 2" high at 100 yards, the same bullet is down by less than 2" at 200 yards. 

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