This cartridge was developed specifically for use in 1000 yard benchrest competition. It was created by simply necking the 338 Winchester Magnum to 30 caliber and almost exactly duplicates the 308 Norma Magnum (Norma's commercial offering has slightly less case taper and is slightly longer). The 30-338 Winchester Magnum fills a void in Winchester's Magnum line, created when Winchester introduced the 300 Magnum. The 264 Winchester Magnum, 338 Winchester Magnum, and 458 Winchester Magnum all share a 2.5 inch case length. Evidently to avoid direct competition with existing 308 Norma Magnum and to better compete with the well established, and substantially longer, 300 Weatherby Magnum, Winchester opted to increase case length and push the shoulder forward on their new 30 caliber magnum (actual usable capacity increase was marginal). The Wildcat 30-338 is likely exactly what Winchester would have offered had Norma not beaten them to the punch. Ballistics are very similar to the 300 Winchester Magnum, despite the slight reduction in powder capacity. Compared to that commercial chambering, a slightly longer case neck provides superior purchase for longer bullets. 

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