The 300 Pegasus was introduced in 1994 and is based upon an entirely new case that features a 0.580 inch head size. The rim is essentially identical to the 378 Weatherby Magnum case, but the design has no belt. Therefore, case capacity is substantially greater for any given cartridge length.
The 300 Pegasus seems a good choice for those who feel they need a flat shooting round that can deliver substantial energy to targets at long range. In a typical rifle, recoil would have to be classed a bit heavy and barrel life is quite limited. 

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