The .25-243 cartridge, also known as the .25-243 Winchester, is a wildcat round, meaning it is not mass-produced but rather custom-made by modifying existing cartridges. It is based on the popular .243 Winchester case, necked up to accept a .257 caliber (6.5mm) bullet.
This cartridge is designed for those looking to improve upon the ballistics of the .243 Winchester by using slightly larger, heavier bullets for potentially better performance in terms of energy delivery and penetration. It is primarily used in hunting medium-sized game, such as deer and antelope. The .25-243 is appreciated for its flat trajectory and manageable recoil, which makes it favorable for longer-range shooting situations.
Since it is a wildcat cartridge, reloading and custom barrel modifications are typically a requirement, and commercial availability of the ammunition is rare. This also means performance characteristics can vary depending on the specific components and loads chosen by the handloader.
Overall, the .25-243 offers a versatile option for hunters and shooting enthusiasts looking for a balance between the lighter .243 Winchester and the larger .257 Roberts cartridges.