The 7.62x51mm NATO and .308 Winchester cartridges are very similar in appearance and dimensions, and they can often be used interchangeably in firearms designed for either cartridge. However, there are some subtle differences between them, primarily related to chamber pressures and intended use:
- 7.62x51mm NATO: The 7.62x51mm NATO cartridge is designed for military use and is loaded to slightly lower pressure levels compared to the .308 Winchester. This is done to ensure reliability in military rifles and machine guns, which can be subjected to harsh conditions.
- .308 Winchester: The .308 Winchester is a commercial cartridge developed for civilian use. It is typically loaded to higher pressures than the 7.62x51mm NATO, which can result in increased velocity and energy.
- 7.62x51mm NATO: Military firearms chambered in 7.62x51mm NATO often have longer chambers with more generous headspace tolerances. This is done to accommodate a wide range of ammunition and ensure reliable function even with slightly out-of-spec rounds.
- .308 Winchester: Commercial firearms chambered in .308 Winchester typically have chambers with tighter headspace tolerances to enhance accuracy and safety when using commercial ammunition. As a result, it's generally not recommended to fire commercial .308 Winchester ammunition in military surplus rifles chambered for 7.62x51mm NATO, as it may lead to excessive pressures.
- Both cartridges are available in a wide variety of loadings, including different bullet types and weights for various applications such as hunting, target shooting, and competition.
- Commercial .308 Winchester brass may be slightly thicker than military 7.62x51mm NATO brass. This can affect case capacity and pressure levels when reloading.
SAAMI vs. CIP Standards:
- The .308 Winchester is produced to SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) standards, which may differ slightly from the CIP (Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives) standards used for the 7.62x51mm NATO. This can affect chamber dimensions and pressure ratings, although the differences are generally small.
In practical terms, many modern firearms chambered for .308 Winchester can safely chamber and fire 7.62x51mm NATO ammunition, and vice versa. However, when considering interchangeability, it's essential to consult the manufacturer's recommendations for the specific firearm in question. Older surplus military rifles chambered in 7.62x51mm NATO may have looser tolerances, making them more accommodating to both types of ammunition. Nonetheless, for the best performance and safety, it's advisable to use ammunition that matches the chamber markings on your firearm.