NATO vs. 308 Winchester ammo: What’s the difference

7.62 NATO vs .308 Winchester ammo: What’s the difference?

If you have a modern .308 Win. rifle, you should be fine firing any modern commercial .308 Win. or 7.62 NATO ammo.

By Brian Smyth | Published Mar 29, 2021 12:55 PM

  • Military Tech
  • Gear

Do a search on the internet for “7.62×51,” and you will get results for “7.62 NATO,” “.308 Winchester,” and multiple variants thereof. Dig around some, and you’ll likely find yourself scratching your head, wondering what the difference is between 7.62 and .308 rounds.

Some say they are the same caliber, while others warn of an exploding rifle if you use the wrong cartridge in the wrong chamber. Do a search for “.308” and you will find similar results, albeit with fewer references to 7.62. So, what gives? Are 7.62 NATO (a.k.a. 7.62x51mm) and .308 Winchester the same thing? Or are they different? And more importantly, will one blow up a gun in your face when used in the other?

Fear not. Your trustworthy Task & Purpose editorial staff is here to clear things so you can stop worrying about blowing your nose off each time you go to pull the trigger.

What are 7.62 NATO and .308 Winchester?

7.62 NATO (a.k.a., 7.62x51mm) and .308 Winchester are two very common rifle cartridges commonly used by U.S. service members and civilians.

The U.S. military’s 7.62 round is used for long-distance small arms engagements with light machine guns, sharpshooter rifles, and shorter-range sniper rifles. While the .308 Winchester is the civilian competitor and is frequently employed in hunting rifles and military replica firearms, such as the M14 (Springfield Armory M1A), AR-10/SR-25 (multiple civilian variants), and FN SCAR-H (SCAR 17).

7.62 NATO vs .308 Winchester ammo: What’s the difference?

A 7.62×51 NATO (left) and .308 Winchester (right) (Brownells)

The history of 7.62 NATO and .308 Winchester

Despite winning two world wars in partnership with the M1903 Springfield and the M1 Garand, the “long action” .30-06 Springfield fell out of favor with the U.S. Army following World War II. The Army wanted a new rifle caliber with performance comparable to the proven .30-06 but with a shorter overall length, a new wonder cartridge. While the .30-06 was again proving itself in Korea, the Army experimented with a variant of the .300 Savage that is designated the T65.

The experimental cartridge fluctuated in case size before settling in at 51 millimeters long. With the .30 caliber bullet diameter being translated as “7.62 millimeters”, the new 7.62x51mm cartridge was born.

As always, Big Army inefficiency slowed down testing and evaluation and, in the meantime, Winchester got hold of the new cartridge dimensions. In 1952, Winchester beat the Army at its own game and launched the new “.308 Winchester” to the civilian market, a good two years before the 7.62×51 saw the Pentagon’s official green light.

What’s the difference between 7.62 NATO and .308 Winchester?

While 7.62 NATO and .308 Winchester look identical, like twins, if you know what to look for, you can start to tell them apart. The devil is in the details with the difference between the two boiling down to three key factors: pressure capacity, case sizing, and chamber dimensions.

NOTE: Many shooting newbies may see the number combination “7.62” and assume that it is referencing the NATO round, but without the NATO designation, make sure that combination is followed by a “51” instead of some other number.

All 7.62×51 rounds are cool for use in the proper NATO weapon, but 7.62x39 is the original caliber of the infamous AK-47. The new(ish) .300 Blackout bears a similar (albeit rare) designation of 7.62×35, and the standard Russian sniper round is the 7.62x54R. Need help keeping it straight? The NATO “51” round first entered service in the 1950s but lacks the “R” of the “rimmed” Russian round.

Task & Purpose glossary for 7.62 NATO and .308 Winchester

Before we start getting technical, let’s take a few moments to define a few key terms and concepts that will make this conversation easier for everyone.

Cartridge

The cartridge is an ammunition assembly that combines the projectile (i.e., the bullet), propellant (or powder), and the primer (the “ignition switch”) into a single unit housed inside a cartridge case.

The cartridge case (a.k.a. “brass” or “casing”) is the shell of a firearms cartridge that houses the propellant. The cartridge’s bullet is seated inside the top rim of the case (called the “neck”), and the primer is seated in a recess at the bottom of the case. Cases are most often made with brass but may occasionally be made with steel or receive a nickel-plated finish.

Chamber

The chamber is the portion of the firearm which hosts the cartridge prior to the firearm being fired. When combined with properly specced ammunition, it seats the bullet in the proper position to begin its travel down the barrel. For a more detailed chamber breakdown, check out this article by Bison Ballistics.

Headspace

Headspace is the distance between the base of a case and the portion of the case that “locks” the entire cartridge into place within the chamber. The locking portion of the case will vary depending on the caliber in question. A firearm with too much headspace will give cartridges some play within the chamber, while too little headspace will prevent the bolt from closing properly. Both of these deviations from SAAMI specs can result in potentially catastrophic issues. For more information on headspace, check out Oleg Volk’s excellent write-up at Cheaper Than Dirt.

SAAMI

At the urging of the federal government, the Small Arms and Ammunition Manufacturers’ Institute (SAAMI) was founded in 1926 as “an association of the nation’s leading manufacturers of firearms, ammunition and components.” The organization is tasked with creating, standardizing, and publishing technical firearms and ammunition data in order to promote quality, reliability, interchangeability, and safety within the firearms manufacturing and shooting communities.

Shoulder (cartridge)

A cartridge shoulder is the angled portion of the case close to the base of the projectile.

Tolerance

Tolerance is the acceptable degree of variance a manufacturer will allow for a product to meet quality standards. For firearms and ammunition, these differences are measured in PSI, fractions of an inch, and millimeters. “Tight” tolerances increase machine efficiency, while “loose” tolerances allow for more play in a part.

Screwing on the pressure

First, let’s talk about pressure. When you pull the trigger on your M14, it starts a chain reaction. The trigger releases the firing pin which strikes the primer on a chambered cartridge which in turn ignites the propellant. If you were paying attention in Mr. Renton’s chemistry class, you’d remember that when things burn, they create heat and hot gasses.

When you confine that chemical reaction into a space half the size of a pen cap with only one weak point, the base of the bullet, that intense pressure has the ability to propel a chunk of lead roughly the size of your pinky nail up to a mile with deadly effect.

Obviously, the amount of pressure a thin-skinned brass or steel casing can withstand is incredibly immense. Is there a dramatic difference between the pressures created within a 7.62 NATO case and a .308 Winchester case? No, and yes.

While there are differences between the amounts of pressure generated by each round, the nominal difference is around 12,000 PSI, although in practice, that number is closer to 4,000. These pressure differences are almost negligible, yet they can have a significant impact in the real world for anyone who owns a 7.62-chambered firearm or who likes to reload their own ammo.

Why you ask? Read on, and it will all make sense.

Walls of brass

Contrary to the popular urban legend, 7.62 NATO casings do indeed have thicker walls than .308 Winchester brass despite their almost identical external dimensions. While the difference may feel minimal, this means that the interior of 7.62×51 brass has less space for powder than a visually identical .308 Winchester case.

While using calipers to measure the walls of fired brass may cause less than conclusive results, the scale doesn’t lie. According to multiple non-scientific studies, the average .308 case weighs in around 160 to 170 grains each, while 7.62 brass usually comes in at over 180 grains apiece. (Yes, grains are an actual measurement of weight.)

While this may sound contradictory, it isn’t. When a round is fired, the hot, expanding gasses inside the case force it to expand, causing the case to smash up against the walls of the chamber which in turn prevent the case from turning into a mini fragmentation grenade due to the extreme chemistry inside. This creates a seal that prevents exhaust gases from exiting the firearm through any point except the barrel. Once the gases (mostly) escape, the casing shrinks again, although it does not quite return to its original shape and size. Of course, this all takes place within mere fractions of a second.

Two in one?

While the external dimensions of both 7.62×51 and .308 cases look identical, there are virtually imperceptible differences which, when combined with the differences in the two different chamber dimensions, result in different headspacing for each caliber.

The 7.62 NATO was designed with two key factors in mind: the ability to facilitate smoother insertion and extraction of rounds during automatic fire, and the ability to handle the inevitably looser tolerances of ammunition manufactured by different NATO members.

The 7.62×51 NATO chamber is imperceptibly longer than the .308 Winchester, extending between 0.006 and 0.010 inches beyond the .308 SAAMI specs. This results in a 7.62 case that is ever so slightly longer than its commercial brother. This means that when firing a .308 round in a 7.62 chamber, the .308 brass will stretch beyond its intended elastic range. This could have some potentially expensive consequences.

So can you shoot 7.62×51 in the same rifles as .308 Winchester? And vice-versa?

Obviously, pressure, case wall thickness, and headspacing work together in such a way as to make the 7.62×51 NATO and the .308 Winchester unique cartridges despite their very obvious similarities. So, the natural question is this: can you shoot 7.62 ammo through a .308-chambered firearm? Is the reverse possible? The short answers to these questions are “yes” and “maybe”.

The vast majority of rifles, carbines, and pistols chambered in .308 Winchester can safely sling 7.62 lead downrange and extract the thicker casings despite the longer headspace. That said, we wouldn’t make a habit of it. Doing the opposite is a bit more complicated.

7.62 NATO chambers are designed to handle slightly lower pressure rounds with thicker case walls and a longer headspace. This means that when you pull the trigger, 7.62×51 brass expands less outwardly but farther toward the chamber. When you switch this out with a .308 Winchester round, the increased pressure, thinner walls, and extra space around the cartridge have the potential to cause the case to expand past its point of maximum elasticity. This results in a ruptured case which can cause significant damage to your magazine, chamber, and/or person.

While most modern 7.62×51-chambered firearms are specced to safely handle .308 cartridges, older NATO-chambered guns are not. This means throwing .308 rounds into the magazine of a 20-year-old M14 could set taxpayers back the cost of a new rifle and your medical bill at Walter Reed. If you have a newer 7.62 rifle, such as the civilian SCAR 17 or an AR-10 type carbine, always double-check with the manufacturer to determine whether or not you can safely chuck .308 rounds downrange. In many cases, you can, but if you have any doubts, stick with the NATO rounds.

Conclusion

Like the 5.56 versus .223 comparison, understanding the differences between 7.62×51 NATO and .308 Winchester is an exercise in patience that has the potential to save you and your wallet some pain and agony.

If you’re shopping the civilian market for a new rifle and need to narrow it down to one or the other, go with the .308 Winchester option. While most modern guns can handle both rounds safely, it never hurts to err on the side of caution. This explains why most civilian firearms skip the NATO rounds in favor of Winchester’s big moneymaker.

FAQs on 7.62 vs. .308

Task & Purpose’s additional intel brief.

Q. Is there a difference in performance between the 7.62 NATO and .308 Winchester?

A. Maybe ask the doctorate students at MIT. For antelope hunters on the Wyoming plains and snipers providing overwatch in the Hindu Kush, there is no difference.

Q. What is the max effective range of the 7.62 NATO and .308 Winchester?

Q. Why is .308 so popular?

A. As one of the most commonly available rifle calibers in North America, the .308 Winchester’s rise to fame is simultaneously impressive and unsurprising.

When the U.S. Army began developing the 7.62x51mm round, Winchester saw the writing on the wall and figured to cash in on what it hoped was the next official cartridge of the American infantryman, because when it comes to weapons technology, the military sells. In addition to riding on the back of the 7.62 NATO’s short-lived military career, the .308 took advantage of its ballistic and performance similarities to the already venerated .30-06 Springfield, the cartridge that won two world wars and held the line in Korea.

Q. Do snipers use 7.62×51 NATO?

A. While U.S. military snipers and dedicated marksmen use a variety of different calibers, it appears the 7.62×51 is here to stay for the foreseeable future.

Video on 7.62 NATO vs. .308 Winchester

Looking for a two-minute summary of the differences between 7.62x51mm NATO and .308 Winchester? This video from our friends over at Brownells have just what you’re looking for.

Our favorite 7.62 and .308 Related Products

Looking for the perfect accessories to accompany your new .308 or 7.62×51 rifle? There are plenty of options from which to choose. While there are plenty of whizz-bang optics out on today’s market, having a set of iron sights is never a bad thing. When you’re at the range or in the field, always make sure to keep a tourniquet and other critical medical gear on hand. Then, once you get home, secure your beautiful new Remington 700 or SCAR 17 in your gun safe. Stay safe, y’all!

Task & Purpose and its partners may earn a commission if you purchase a product through one of our links.

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