The question of whether .308 Winchester and 7.62x51mm NATO rounds are interchangeable is a recurring debate among firearm enthusiasts. Often compared alongside discussions about other calibers like the .556 NATO, understanding the nuances between these cartridges is crucial for safety and performance. While they are frequently treated as ballistically similar and functionally interchangeable in many modern firearms, subtle yet significant differences exist, primarily in their chamber specifications and pressure levels. This article delves into these distinctions to clarify the relationship between .308 Winchester and 7.62x51mm NATO.
The Confusion: Are They Really the Same?
The common misconception is that .308 Winchester and 7.62x51mm NATO are identical, and in practice, for many shooters using modern rifles, this often holds true. You’ll often hear arguments claiming they are “the same” or “not the same,” contributing to the ongoing confusion. The reality is more nuanced than a simple yes or no.
Historical Context: Civilian vs. Military Standards
The .308 Winchester cartridge was introduced to the civilian market by Winchester Ammunition in 1952. Shortly after, the military adopted a cartridge of similar dimensions, the 7.62x51mm NATO, as the T65E3, standardizing it for military applications. The .308 Winchester essentially served as the basis for the 7.62x51mm NATO. However, standardization processes and differing priorities between civilian sporting cartridges and military ammunition led to variations in specifications, particularly concerning chamber pressures and case toughness.
Chamber Specifications and Pressure Differences
The most critical difference lies in the chamber specifications defined by SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) for .308 Winchester and military specifications for 7.62x51mm NATO. While cartridge dimensions are very similar, the maximum average pressure (MAP) ratings differ. SAAMI specifies a higher MAP for .308 Winchester compared to the military 7.62x51mm NATO.
This pressure difference stems from different testing methodologies and design priorities. Military ammunition is often designed to function reliably in a wide range of conditions and firearms, sometimes prioritizing reliability over ultimate accuracy or pressure limits found in commercial sporting ammunition. Military chambers may also be slightly more generous in dimensions to enhance reliability under adverse conditions, which can contribute to perceived headspace differences.
An M14 rifle, commonly chambered in 7.62x51mm NATO, highlighting the military context of this cartridge. The robust design of military rifles often accommodates the slightly different specifications of 7.62 NATO compared to commercial .308 Winchester.
Interchangeability: Practical Considerations
In modern rifles chambered for .308 Winchester, it is generally safe to fire 7.62x51mm NATO ammunition. The slightly lower pressure specification of 7.62 NATO means it will operate well within the pressure limits of a .308 chamber.
However, the reverse situation warrants more caution. Firing commercial .308 Winchester ammunition in rifles specifically chambered for 7.62x51mm NATO, especially older military rifles, could present issues in certain scenarios. Some older 7.62 NATO chambers, particularly those manufactured to looser military specifications, might have longer headspace. While most modern 7.62 NATO rifles are built to tighter tolerances, the concern primarily arises with older rifles like some military surplus rifles.
Walt Kuleck from Fulton Armory notes that “in some 7.62MM rifles the chambers are long (to the 7.62MM military spec), notably the Navy Garands with 7.62MM barrels. Thus, using commercial ammo in such a rifle is not a good idea; you need stronger brass. Use military ammo or the best commercial only, e.g., Federal Gold Medal Match.”
Clint McKee of Fulton Armory also emphasizes the importance of headspace, stating that “if you have a chamber with headspace much in excess of 1.636 (say, 1.638, SAAMI field reject), you must use only U.S. or NATO Mil Spec Ammo… since the NATO mil spec calls for a far more ‘robust’ brass case than often found in commercial (.308 Winchester) cartridges.”
For modern rifles in good condition and within SAAMI headspace specifications, the interchangeability is less of a concern. However, when dealing with older military rifles or rifles with unknown headspace, caution is advised. Checking headspace with gauges is recommended, especially before using commercial .308 ammunition in older 7.62 NATO rifles.
.308 vs .556 NATO: A Different Comparison
While this discussion focuses on .308 and 7.62x51mm, the .556 NATO (.223 Remington) is another common cartridge often brought into the conversation, particularly when discussing military and civilian ammunition. However, the .556 NATO is a significantly smaller cartridge than both the .308 Winchester and 7.62x51mm NATO.
The .556 NATO is designed for lighter recoil and higher velocities, typically used in rifles like the AR-15 platform for infantry and close-quarters combat. In contrast, the .308/7.62 cartridges offer greater range, penetration, and stopping power, suitable for longer-range engagements and larger game hunting. The comparison between .308 and .556 is less about interchangeability and more about different applications and performance characteristics. It is absolutely critical to note that .556 NATO and .223 Remington are also not fully interchangeable and firing 5.56 NATO in a rifle chambered for .223 Remington can be dangerous due to pressure differences, similar to, but distinct from, the .308/7.62 discussion.
Conclusion: Understanding and Caution
In summary, while .308 Winchester and 7.62x51mm NATO are very similar and often interchangeable in modern firearms, they are not identical. The key differences lie in chamber pressure specifications and, potentially, headspace in older military rifles. For most contemporary .308 Winchester rifles, using 7.62x51mm NATO ammunition is generally safe. However, caution is advised when using commercial .308 Winchester in older rifles specifically chambered for 7.62x51mm NATO, especially those of military origin. Understanding these subtle distinctions and, when in doubt, consulting a gunsmith or performing headspace checks ensures safe and reliable operation of your firearm.
References:
- http://www.thefiringline.com/forums/…threadid=56047
- http://www.thefiringline.com/forums/…threadid=15031
- http://www.saami.org/unsafe3.htm
- http://www.fulton-armory.com/308.htm
