Ammunition can be confusing, especially with rounds that are extremely similar. The .308 Winchester and the 7.62×51 NATO cartridges are two of these types of rounds. This is because they can be easily loaded into a barrel chambered for the other round.

While it is not recommended to cross load these cartridges, the Sporting Arms and Ammunition Manufacturer’s Institute (SAAMI) do not consider it dangerous to load .308 into a 7.62 NATO chamber while the NATO round will feed reliably in a .308 gun. This raises the question “If they are so similar what is the difference?”

This will take a good chunk of time to figure out and cover several decades of ammunition development. So get ready and get comfortable.

What is the difference?

The .308 Winchester cartridge is loaded to higher pressures with a wider range of projectiles than 7.62 NATO, which is standardized for military use.

This is easier to understand with a chart than it is in words. Especially when some loads are designed to be cross-compatible.

.308 Round and Projectile Weight	Velocity
Spitzer bullet 125 gr	3,100 ft/s (940 m/s)
Nosler tip 150 gr	2,820 ft/s (860 m/s)
BTHP* 168gr	2,650 ft/s (810 m/s)
BTHP 175 gr	2,645 ft/s (806 m/s)

*Boat-tail Hollow-point

If you want more loading data check out this link for .308 loads.

7.62×51 NATO Round and Projectile Weight	Velocity
M80 FMJ 147 gr	2,800 ft/s (850 m/s)
M118 long range BTHP 175 gr	2,600 ft/s (790 m/s)

If you want a discussion on 7.62 NATO load data try this discussion here.

But all of this doesn’t tell us how we got to this point. For that, we will look into the early developments of both of these rounds to see what makes them so different.

The Origins of .308 and 7.62 NATO

We will travel back to the early 1900s. Various conflicts and eventually World War I forced the United States military to adopt a smaller bore bolt-action rifle. At the same time, a new cartridge had to be adopted and modified as requirements changed and failures arose.

This resulted in the famous .30-06 cartridge adopted in 1906, which would carry the US Military through the Korean War as its primary infantry cartridge. While it did perform well, this did not stop the US Military from shopping around to find something better.

The .30-06 was hampered in its development by the technology of the time. Over 30 years after it was developed technology had advanced enough to possibly find a better round. This search resulted in the T65 experimental cartridges.

The T65 series was developed to mirror or exceed the performance of .30-06 and used the commercial .300 Savage cartridge as its basis. The .300 Savage was able to propel a 150 grain projectile at speeds exceeding 2,600 ft/s while still being effective at ranges over 300 yards.

While this range was less effective than the original .30-06 it was still a good cartridge to try and modify to achieve a smaller overall cartridge with similar ballistic performance.

There was one drawback. The Savage cases had thinner walls than the T65 cases. These T65 cases were made by Frankford Arsenal with standard .30-06 Springfield cases. This resulted in a lower case capacity.

To circumvent the lower internal capacity the T65 case was lengthened to meet the .30-06 performance with, at the time, modern propellants. This resulted in a 7.62×51 cartridge that was more than 10 mm shorter than the .30-06 cartridge with less capacity but similar performance.

By this time the US Military was already trying to increase the firepower of the individual soldier by providing each with a rifle capable of semi-automatic and automatic fire. Automatic rifles were tried with .30-06 but were found to be too uncontrollable.

This caused a renewed interest in a new cartridge which coincided with the completion of the T65 trials. This final iteration of the T65 cartridge was the T65E5 which marks the split between the .308 and the 7.62 NATO rounds.

.308 Winchester

Winchester recognized the potential of the T65E5 cartridge and decided to bring it to the civilian market in 1952. This was roughly two years before the T65E5 was adopted by NATO.

The civilian market loved it. The ballistic similarities between .308 and .30-06 allowed it to be used in a wide variety of hunting activities. Even when compared with .300 Winchester Magnum, which has better long-distance performance, the differences under 300 yards are minimal. 

Even with long range shooting, the .308 cartridge has still found a place in the civilian market. This is because of even more advances in loading technology combined with a variety of projectiles.

The right loadings can push a 110 grain projectile at 3200 ft/s while most loadings will fall between 2400 and 2900 ft/s depending heavily on the bullet weight and the propellant charge used.

The projectile weights range from 110 “varmint” projectiles to 210-grain bullets. With the modern market, there is such a wide variety of .308 options you will be able to find any combination you want or need for the task you want.

This is one of the major differences between .308 and 7.62 NATO. The .308 is designed to appeal to a wide audience and provide everything from taking a medium-sized game to 1000 yard shots depending on the load selected.

 The other major difference is actually a very small one. This is found in the headspace and chamber size of the .308 and 7.62 NATO.

The .308 has a headspace of roughly 1.630” which can be 0.013” smaller than the NATO round’s headspace. This difference can cause .308 to experience case issues if it is at its maximum pressure in a chamber with a headspace that is too long for it.

As a final consideration, the .308 cartridge has thinner case walls than 7.62 NATO and can have a hotter loading than most NATO offerings. Sometimes as much as 12,000 PSI is more than a standard 7.62 round.

This additional pressure can run the risk of damaging the gun and the user. This is why most manufacturers recommend only shooting ammunition designed for your barrel chambering.

This brings us back to the development of 7.62 NATO. 

7.62x51mm NATO

While Winchester was releasing the T65E5 on the civilian market a secondary thread of development was going on throughout the world.

Like the early .30-06 automatic rifle tests of the US, in most countries around the world we’re finding that a full-powered rifle cartridge was uncontrollable in full auto. This would lead to a variety of one-off intermediate cartridges being developed with the UK leading the charge.

However, there was the question of standardization of ammunition among the NATO allies at this time. The logistical nightmare of multiple countries each having their own unique ammunition and rifles with no compatibility had already been a reality in the First World War.

NATO hoped to avoid this and set out to find a standard round to use. At the time, the US was wrapping up their T65 trials and would not accept a round smaller than .30-caliber or 7.62 mm.

This ultimately resulted in the adoption of the T65E5 as the 7.62x51mm NATO standard cartridge in 1954. During this time the US was attempting to improve and replace the M1 Garand which they would do in 1957.

Part of the process was to develop a new round that would be used by both the infantry’s rifles and the standard machine guns in use at the time. The .30-06 had filled this role well and the US hoped that the 7.62 NATO cartridge would also fill both roles in time.  

At roughly the same time the M14, the FAL, and the G3 were adopted by the US and various NATO allies. They quickly found out that 7.62 NATO was uncontrollable in full-auto fire. Resulting in most of the rifles chambered in it to be modified in some way to be either select fire or semi-auto only.

For the US, the 7.62 NATO round would continue on in the hands of the infantry until 1964 when experience in Vietnam showed it was not as ideal for that combat setting as most had hoped. From then on it became a special use round or a machine gun round.

Because of this, the cartridge had to be able to work well in a variety of chamber sizes. This resulted in the case being thicker than the .308’s in order to allow proper expansion in the chamber.

The pressure limits also had to be standardized in order to create a round that would work in any gun it was chambered for without fine-tuning the ammunition to that particular gun. 

Today it is mostly used as a specialty round since most militaries have adopted an intermediate cartridge as their infantry round of choice. This means military 7.62 NATO is being made similar to more match grade .308 in order to get more accuracy out of it.

However, this creates even more confusion when trying to find loading data for 7.62 NATO or .308 rounds.

A good rule to follow is that 7.62 NATO tends to need 10% less propellant than .308 or experimentation should start at 10% less and slowly be worked up to your desired velocities. Be careful to keep an eye out for failing brass or other stressors to keep the pressure at a safe level.

Before we get into some safety information let’s look at the differences between these two cartridges again.

Cartridge Differences and Dimensions

(Image is 7.62 NATO dimensions)

When you look at the nitty-gritty of these cartridges there is a lot that looks the same. This can lull us into a sense of complacency which is dangerous when we’re dealing with firearms.

A lot of effort goes into developing each of these rounds so that they can be repeatedly manufactured. This means every detail matters especially for SAAMI accepted rounds.

Both of these rounds are proofed to ensure their durability and safety. This proofing was done by one of three entities. One is SAAMI, which we’ve already mentioned enough. The others are the NATO EPVAT testing procedures and the C.I.P.

The EPVAT covered 7.62 NATO specifically but did not have anything to do with the .308 cartridge. If you want to look through their entire brief on 7.62 NATO you can do so here.

The final entity to proof both cartridges is the C.I.P. (Commission Internationale Permanente pour l’Epreve des Armes à Feu Portatives).

Each of these different testers found both rounds can handle slightly over 75,000 psi pressures for the testing round without a catastrophic failure. However, the official pressure ratings are significantly lower than this. The official rating for .308 being 60,191 psi according to the C.I.P. and 62,000 psi according to SAAMI while 7.62 NATO’s ratings is 60,191 according to the NATO EPVAT and C.I.P.

(Image is .308 dimensions)

Due to the civilian nature of .308, it is more likely to be experimented with, which means the pressure limits are more likely to be pushed in a non-testing environment. Hotter loads for specialty tasks or preferences will push the ratings for the round.

The 7.62 NATO round on the other hand needs to be as consistent as possible for use. Most variants of the round are for training or testing equipment. With M80 Ball and M80A1 being the more common round with the M118LR being used for longer range accuracy.

Let’s look at the individual dimensions of each cartridge.

The .308  has a bullet diameter of 0.308 in. (7.8mm), a neck diameter of 0.3433 in. (8.72mm), and a shoulder diameter of 0.4539 in. (11.53mm). The base diameter is 0.4709 in. (11.96 mm) while the rim diameter is 0.4728 in. (12.01mm) with a thickness of 0.0539 in. (1.37mm). Overall the case is 2.015 in. (51.2 mm) and the entire cartridge’s overall length is 2.800 in. (71.1 mm).

The 7.62 NATO round is 2.800 in (71.1 mm) overall with a case length of 2.015 in. (51.2 mm). The rim diameter is 0.473 in (12.0 mm) with a thickness of 0.050 in. (1.3 mm). The base diameter is 0.470 in. (11.9 mm). The case shoulder is 0.454 in. (11.5 mm). The neck is 0.345 in. (8.8 mm) and the bullet diameter is 0.308 in. (7.82 mm).

As you can see the differences are extremely small. But text blocks don’t convey that as easily as a chart.

Dimensions	.308	7.62 NATO
Bullet diameter	0.308 in. (7.8mm)	0.308 in. (7.82 mm)
Neck diameter	0.3433 in. (8.72mm)	0.345 in. (8.8 mm)
Shoulder diameter	0.4539 in. (11.53mm)	0.454 in. (11.5 mm)
Base diameter	0.4709 in. (11.96 mm)	0.470 in. (11.9 mm)
Rim diameter	0.4728 in. (12.01mm)	0.473 in (12.0 mm)
Rim thickness	0.0539 in. (1.37mm)	0.050 in. (1.3 mm)
Case Length	2.015 in. (51.2 mm)	2.015 in. (51.2 mm)
Overall Length	2.800 in. (71.1 mm)	2.800 in (71.1 mm)

These rounds are different by a mere fraction of an inch or millimeter. This shouldn’t be surprising since they both came from the same development trial.

The line between .308 and 7.62 NATO is blurred even more today with specialized loadings and more neutrally cut chambers in the civilian market. This makes the rounds almost interchangeable.

While the C.I.P. and SAAMI testings show that it is not unsafe to fire a .308 out of a 7.62 NATO chamber it is still a matter of discussion among gun enthusiasts. And it really comes down to how confident you are in the ammunition and gun you are using.

Safety Sidenote

Before we conclude it should be mentioned that safety should always be a factor in what we load into our guns.

You will have noticed throughout this entire article the military round is either referred to as 7.62 NATO or 7.62x51mm. This is because there are many 7.62 or .30 caliber rounds.

The .30-06 cartridge is also known as 7.62x63mm and will not fit in the action of either a .308 or 7.62x51mm chamber. The AKM’s round is a 7.62x39mm cartridge. There is even a pistol round that is a 7.62 caliber (7.62×25 Tokarev).

Make sure the dimensions of your bullets match the caliber of your gun to avoid frustration and trying to feed the wrong bullet in the wrong gun.

Conclusion

The .308 and 7.62 NATO cartridges are extremely similar. It all comes down to their integral dimensions and how they are loaded. Today they have roughly harmonized with each other to the point where they are easily interchanged.

While this is not recommended it should not matter if you have a quality firearm to put them through.

This makes reloading and purchasing new ammunition a little confusing. However, the best practice for any loading data is to start 10% lower than what the recipe states just to make sure it doesn’t explode.

For modern manufactured ammunition, it is much simpler. Match the round to your barrel and you will be fine. If one or two alternative rounds make it in without you noticing, it should not be dangerous. However, if you choose to use them interchangeably keep in mind the wear on the gun might be different.

Keep an eye on your gun’s components for potential fractures or failure points. If they develop, stop using the gun immediately until the appropriate parts have been replaced.