The 6.5-300 Weatherby Magnum is an extremely high-velocity 6.5mm cartridge based on the .300 Weatherby case necked down to 6.5mm. Standardized in 2016, it is among the fastest 6.5mm cartridges made, built for flat-shooting long-range performance.

The 6.5-300 Weatherby Magnum was a wildcat concept for decades before Weatherby standardized it as a factory cartridge in 2016. It takes the large .300 Weatherby case and necks it down to the aerodynamic 6.5mm (.264 caliber) bore, creating an "overbore" cartridge with very large case capacity behind a relatively small bullet — the recipe for extreme velocity. Ballistically the 6.5-300 Weatherby is among the fastest 6.5mm cartridges available, driving aerodynamic 6.5mm bullets at exceptional velocity for a very flat trajectory and strong long-range performance. It essentially sits at the maximum-velocity end of the popular 6.5mm spectrum — far faster than the 6.5 Creedmoor or 6.5 PRC. The tradeoffs are pronounced: significant recoil and muzzle blast, and notably accelerated barrel wear from the overbore design, which long-range shooters weigh against its flat-shooting reach. It appeals to shooters specifically seeking maximum 6.5mm velocity and long-range capability, occupying a clear niche above the more efficient, milder 6.5mm cartridges. Its ammunition is specialized and premium-priced, consistent with the Weatherby line. Today the 6.5-300 Weatherby Magnum is used for long-range target shooting and big-game hunting, with availability centered in Weatherby rifles and ammunition. Its standing as one of the fastest 6.5mm cartridges, with an extremely flat trajectory, is why it draws shooters pursuing maximum reach in that bore.

General information about this caliber — not a recommendation for this specific firearm.