The 25-303 cartridge is a wildcat round, created by modifying the standard .303 British cartridge case to accept .257 caliber (6.5mm) bullets. This involves necking down the .303 British case to the smaller diameter. The 25-303 is relatively obscure and not commonly found in commercial production; it's often hand-loaded by enthusiasts who are looking for a unique cartridge with good ballistic performance for hunting or target shooting.
The cartridge was developed to provide shooters with a flatter trajectory and higher velocity compared to the original .303 British round, making it suitable for medium game hunting in regions where the .303 British is traditionally used. Given its wildcat status, users typically have specialized reloading equipment and knowledge.