To give you a better sense of how the 6.5-300 Weatherby Magnum measures up against other 6.5mm calibers on the market, take a look at this graph. This shows how the three factory loads that Weatherby is unveiling—the 127-gr. Barnes LRX, the 130-grain Swift Scirocco, and the 140-grain Swift A-Frame—compare to some heavy-for-caliber loads in the 26 Nosler, the 264 Win. Mag., and the 6.5-284 Norma.
Based on this info you can see why Weatherby is confident claiming that the 6.5-300 is the fastest 6.5 cartridge on the planet.