Can You Shoot a 7.62 x 51 Ammo in a .308 Rifle?

Introduction

The straight answer to this question is yes, you can fire common 7.62X51 military ammo in any currently produced rifle chambered for .308 Winchester. However, there is more than one reason for this evaluation. 

The 7.62×51 NATO cartridge is specific and it is built for military specifications. The reason is to make the round able to operate both belt-fed machine guns and semi-automatic rifles. The current specification for 7.62×51 is a pressure limit of 50,000 psi. And it is best for operating the M14, machine guns, and other semi-auto gas guns that the military uses.

As for the .308 Winchester ammunition, the pressure limit is SAAMI standardized at 62.000 psi. It makes a slight difference in cartridge specs between the military round and the .308 Winchester as currently produced. Therefore, the basic understanding is that the military cartridge will fit and fire in currently produced .308 rifles.

Normally, in a typical gun saloon bar discussion, the most heated debate is asking a bunch of people about the differences between .308 Winchester and 7.62x51mm NATO.  There is a variety of responses that often include 

• They are not different
• There is a difference
• They might be different

Weirdly all those statements are all quite true. As it may be confusing to grasp, this article will try to explain it in a way that you will not bash your brains out. I will try to be void of making you read all the nuances of things like piezo transducers.

There are only limited categories of comparison and this will pretty much mess things up.

Understanding the Difference Between 7.62×51 and 308 Winchester 

Before we delve much further, I must address this often-posed question “Are the 7.62×51 NATO and .308 Winchester the same?” 

Again, the answer is NO.  It is because of the differences in maximum pressures and the chamber specs. Nobody makes a 7.62mm (NATO) ammo that is not fitting to the .308 ‘headspace’ dimension spec. As a rule, this makes the 7.62mm ammo fit into .308 chambers. 

But if you go the other way, you can encounter problems.  A commercial .308 Winchester round exceeds the maximum rated pressure for the 7.62×51 ammo. If this is the case, avoid putting full-power .308 Win rounds into military rifles designed for a maximum pressure of 50,000 psi.

7.62 vs .308 – Pressure

The pressure is where most people get confused when analyzing the two ammo. The maximum pressure numbers for the two calibers are 50,000 psi for 7.62 and 62,000 psi for .308. At a glance, it appears to be a big difference and a reason to consider it unsafe to fire. The logic is, do not fire a higher pressure commercial .308 cartridge in a rifle built for 7.62x51mm NATO.

While it is true that a .308 ammo has a slightly higher pressure than 7.62x51mm NATO, the reality is that the brass and chambers are the issues to understand.

Here you must concentrate on the details. I put it that the 50,000 number is an accurate representation of copper units of pressure or CUP. It is a less precise way to measure pressure, as the method relies on looking at how much little copper disks compress when you fire the gun.

As things are, there is no consistent mathematical formula that equates CUP to pounds per square inch (PSI). The difference, therefore, in this specific case lies somewhere around 8,000. What this implies is that the maximum pressure for 7.62x51mm NATO is about 58,000 psi and not so far from 62,000 figures for the .308 Winchester.

If both loads are routinely proof tested at far higher levels, the 4,000 difference will not alter the trajectory.

7.62 NATO vs .308 Winchester – Case thickness

Measuring the thickness of the cartridge is quite challenging. I have developed a simpler way of doing it and it helps explain the difference. The process involves the representation of all sets of ammo. From my big bucket selection, I took both the .308 Winchester and 7.62x51mm NATO brass and weighed them. I further picked several of each from different brands and averaged the weights. I could not measure the cases because I had not fired them yet. And also because it will not tell me much other than the general size of the chamber from which they were fired. 

The Winchester .308 weighs 163.3 grains while the Lake City 7.62x51mm NATO weighs 183.5 grains. As you can see this is a significant difference. When the thicker walls combine with similar exterior dimensions this means less powder capacity and the result is a lower- top end. All else the same, the result is a lower velocity and pressure.

The thicker brass of 7.62 cases is a significant factor and you will understand it better in what we discuss next.

7.62 NATO vs .308 Winchester – Headspace

The headspace is where we will get to the real difference. Military rifles for 7.62x51mm NATO usually do have longer chambers. There is a need for reliable feeding and there can be slack if you consider firing a machine gun powered with ammo made from all over the world. The operation needs to be smooth considering all the force used between the feeding and ejection process. 

The solution is making the chamber headspace a bit longer. The headspace is the distance from the bolt face to the point in the chamber that stops the forward motion of the cartridge case. Therefore if headspace is too small, the bolt will not close properly or will require excess force to close. And if the chamber headspace is too long for a cartridge, it will float back and forth in the chamber.  

The .308 Winchester chamber headspace is between 1.630 and 1.6340 inches. While that for 7.62x51mm NATO is between 1.6355 and 1.6405 inches. The slight difference in size might not seem like a big deal until you fire a thinner brass in a long-chambered rifle. The disaster is that the brass will stretch, perhaps enough to lead to a dangerous case rupture. 

However, if you try the same with a thicker military brass, it is no big deal because of the way the system is designed.  Using a thicker brass makes it handle the extra stretching into a longer chamber throat. The solution to the title question is to know your rifle and what its headspace is. Once you know this info you will determine if it is safe to shoot a .308 Winchester ammo in a 7.62 chamber.

How Can I Know if My Headspace is Safe?

To know if your headspace is safe is not a challenging task. You can use the set of .308 Winchester Go / No-Go headspace gauges. Use the gauges to check the chamber size. The bolt should close easily on a Go gauge and not close on a No-Go gauge.  Also, the field gauge checks the maximum published chamber size. With some 7.62 rifles, you might find that the bolt still closes on the No-Go gauge. As long as the bolt closes, you are still within maximum published limits.

The Technicalities

In terms of specifications, there are differences, more so in the chambers of rifles designed to fire each cartridge. For the 7.62 brass is a bit thicker, and with the .308 is sometimes loaded to be slightly higher pressure. Other than that, the cartridges are pretty much the same.

One way to be entirely safe is to remain conservative and only fire 7.62x51m NATO in 7.62 chambered rifles and .308 Winchester in .308 rifles.

A final risk spectrum is just a scenario. In the case of using 7.62x51mm NATO ammo in a .308 chamber. In theory, you might run across particularly long ammo. Such ammo might not chamber or might require undue pressure to the chamber. The result could be dangerously high pressure. While such a scenario is highly unlikely as the 7.62 ammo to be significantly longer, is a pretty rare thing.  It is why not most people will consider it a big deal to use 7.62 ammo in a .308 chambered rifle.

The Reverse Is Not True

However, you need to be careful while using .308 Winchester commercial ammo in a 7.62x51mm NATO chambered rifle. Even though the most modern 7.62 chambers tend to be cut almost closer to match the .308 dimensions, it is always safe to know exactly what you have in terms of headspace. If your rifle has a long headspace, stick to 7.62 NATO ammo. Do not use the .308 Winchester.

Great gear to bring along when visiting the range!

Conclusion

If you put everything else into consideration then the actual pressure differences between the .308 Win. and 7.62 NATO is not that large. However, it is a point of concern. Nonetheless, a tight chamber or an overly dirty chamber could pose a pressure issue. You don’t want to risk damaging a firearm and certainly never wish to see anyone hurt from negligence.  It is why it is important to keep the golden rule in mind. A .308 Winchester chamber rifle can handle both .308 Win. and 7.62 NATO ammo safely, but a 7.62 rifle is only safe for a 7.62 NATO ammo.