The 300 Phoenix is a proprietary cartridge developed by G. McMillan & Co. of Phoenix, Arizona, USA, in 1991. This cartridge is based on the 416 Rigby case necked down to 308 caliber. When loaded with a 250 grs bullet, the overall length is 3.600 inches, which will work trough a standard length bolt action. Anticipated ballistics was 3600 fps with a 180 grs bullet and 3100 fps with a 250 grs bullet. This would have been an outstanding long range, 30 caliber round, suitable for any North American big game at appropriate ranges. However, testing showed that these results were not obtainable. For example, velocities that could be obtained with the 250 grs bullet proved to be only 2800 fps, rather then the desired 3100 fps. Thus, the 300 Phoenix was not a success. 

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