The 25-300 WSM (Winchester Short Magnum) is a wildcat cartridge, which means it is not officially produced by major ammunition manufacturers but rather custom-built by enthusiasts and gunsmiths. This cartridge is based on the commercial 300 WSM case, necked down to accept a .257-inch (6.5 mm) bullet.
The 25-300 WSM is designed to offer enhanced ballistic performance over existing .25 caliber cartridges, including higher velocities and flatter trajectories. This makes it suitable for long-range shooting and hunting medium to large game, such as deer and antelope. The higher velocity and energy delivered by the 25-300 WSM also provide better terminal performance on game animals.
However, as a wildcat cartridge, it requires specialized knowledge and equipment for handloading, and ammunition/components can be harder to source than standard factory-produced cartridges.