What is the Difference Between .308 Winchester and 7.62 NATO?

.308 winchester

Debate rages in the shooting community about the difference between .308 Winchester and 7.62 NATO. Some say it’s insignificant and in name only. Others say it’s so great as to risk danger to life, limb and gun. We’ll examine the facts and by the end, you’ll hopefully be informed enough to make your own decision.

Let’s start with a short history of how these two cartridges came to exist in the first place. After World War I, the U.S Army Ordnance Corps began a search for a smaller cartridge to replace the .30-06 Springfield round that was adopted in 1906. By World War II, not much progress had been made, but in 1944, engineers at the Army’s Frankford Arsenal began experimenting with the .300 Savage case, which Savage Arms developed in 1920 for its Model 99 lever action rifle. Designated Cartridge, Ball, Cal. .30 T65, this experimental round nearly equaled the performance of the larger .30-06 in Ordnance testing. After several revisions, including lengthening the neck of the case, the revised T65E3 cartridge was adopted as 7.62×51 NATO in 1953. Rifles and machine guns chambered in the new round soon began appearing among the Western military powers, including the FN FAL and the US M14 and M60, in 1957.

While the military trials were still in progress, Winchester saw an opportunity in the civilian market and introduced a commercial version of the cartridge in 1952, known as .308 Winchester. Initial public reception was slow due to the popularity of the venerable .30-06, but .308 Winchester has steadily earned a reputation as one of the most accurate and versatile cartridges for hunting and target shooting. In military service, 7.62 NATO continues to be a top choice for sniper and designated marksman rifles thanks to its ability to pack a punch at long distances.

Is there a difference between .308 Winchester and 7.62 NATO?

The external dimensions of the .308 Winchester and 7.62×51 cartridge cases are essentially identical, and with very rare exceptions, the two rounds are physically interchangeable. So are there significant differences between them? The answer is yes. Three important variables separate these two cartridges: headspace, pressure, and case construction.

Long and Short of Headspace

Headspace is the dimension from the face of a firearm’s closed breech to the forward part of its chamber that limits insertion of the cartridge, preventing it from stretching and bursting when fired. Most rimless cartridges like .308 Winchester and 7.62 NATO set headspace on the shoulder of the case between the case neck and cartridge body. 

To promote accuracy, the chambers of commercial hunting and target rifles are generally made closer to the minimum end of the specification range, while military rifles are usually made with larger chambers to ensure reliable feeding and ejection when dirty or during rapid or automatic firing. 

The Sporting Arms and Ammunition Manufacturers Institute (SAMMI), which sets the standards for firearms and ammunition made in the United States, publishes different headspace measurements for .308 Winchester (1.630 – 1.6340 inches ) and 7.62 NATO (1.6355 – 1.6404 inches). That’s a nominal difference of six thousands of an inch, but it’s not uncommon to see some military rifle chambers cut even larger. We’ll soon see why this difference in chamber size is critical to the interchangeability of the two cartridges. 

Under Pressure

This gets a little tricky. SAAMI specifies the maximum chamber pressure generated by .308 Winchester as 62,000 psi. But SAAMI doesn’t control production of military ammunition, especially from foreign sources, and the NATO pressure specification for 7.62×51 is set using a different pressure scale, known as Copper Units of Pressure (CUP). It’s an older standard that measures the deformation of a stack of calibrated copper discs in a pressure test barrel. The maximum pressure for 7.62×51 set by NATO is 50,000 CUP. There’s no direct conversion from CUP to PSI, but testing has shown that maximum loads of 7.62 NATO generate about 58,000 PSI. Bottom line: the difference isn’t great, but 308 Winchester ammunition is generally loaded to higher pressure than 7.62 NATO.

Case Thickness

Another critical difference between the two cartridges is the thickness of military brass compared to the .308 Winchester brass produced by most commercial manufacturers. Military brass cases are made with a thicker case head and side walls, which allows them to stretch more in the larger chambers of military rifles without danger of bursting. As evidence of this difference, most reloading manuals recommend reducing powder charge weights by one to two grains when using thicker military brass to account for its reduced internal capacity. 

Is it Safe to Shoot?

So is it safe to fire 7.62 NATO in a rifle chambered for 308 Winchester or vice versa? In extreme situations, a higher pressure commercial .308 Winchester round with a thinner brass case can stretch enough to cause a dangerous case rupture when fired in a larger 7.62 NATO chamber. This is the most dangerous potential scenario, and although rare, it’s the reason for this entire discussion. To err on the side of caution, only fire 7.62 NATO ammunition in rifles chambered in that round, especially surplus military rifles that are likely to have larger chambers.

On the other hand, it’s rare to have a problem when firing a military 7.62 NATO round in a commercial rifle chambered in .308 Winchester. It’s possible to have difficulty loading military ammunition manufactured at the high end of the NATO headspace specification in a commercial rifle with a minimum-length or match grade chamber. This can cause trouble feeding, closing the bolt or extracting, which is more of an inconvenience than a safety issue. You will also rarely realize the maximum accuracy performance of .308 Winchester from military grade ammunition. 

There you have it. Armed with the facts, you can now choose the best ammunition for your guns and shooting needs, and Patriot Defense has you covered with great choices for 7.62 NATO and .308 Winchester.