7.62 NATO vs .308 Winchester Ammo, What’s The Difference?

Gun nut, Tom McHale, risks a fight to explain the subtle nuances between 7.62 NATO vs .308 Winchester Ammunition.

Can you shoot .308 ammo in a 7.62 rifle? Or is it the other way around?
Can you shoot .308 ammo in a 7.62 rifle? Or is it the other way around?

USA –-(Ammoland.com)- If you want to start a good bar fight, ask about a saloon full of gun people about the differences between .308 Winchester and 7.62x51mm NATO. We’re going to dive into the fray with a simplified and practical explanation. Ready?

  • They are different.
  • They aren’t different.
  • They might be different.

OK, so that was a bit of a wise guy response, but those statements are all quite true. Don’t worry, we’ll explain further. However, we’re going to try to do that in such a way that you don’t want to bash your own brains out by having to read about the nuances of things like piezo transducers.

Let’s look at a few categories of comparison, then we’ll sum things up.

7.62 NATO vs .308 Winchester – History

During the 1940s and early 1950s, military rocket surgeons wanted to improve the effectiveness of the M1 Garand rifle and its .30-06 cartridge. One contender, more suitable for higher capacity box magazines was a modified .300 Savage design called the T65. While specs varied throughout development, it ended up as the earliest iteration of the 7.62x51mm NATO. But, as usual, the commercial market was more nimble, and Winchester launched the .308 Winchester in 1952.

It took the government until 1957 to have a cartridge and rifle hitting the barracks with similar specs.

7.62 vs .308 – Pressure

Here’s where some of the confusion comes into the picture. Maximum pressure numbers thrown around for the two calibers are often shown as 50,000 “psi” for 7.62 and 62,000 psi for .308. At first glance, that appears to be a big difference and a potential reason why folks might consider it unsafe to fire a “higher pressure” commercial .308 cartridge in a rifle built for 7.62x51mm NATO.

While commercial .308 ammo may have slightly higher pressure than 7.62x51mm NATO, it's really the brass and chambers that are the issues to understand.
While commercial .308 ammo may have slightly higher pressure than 7.62x51mm NATO, it's really the brass and chambers that are the issues to understand.

As with most things, the devil is in the details. I put that 50,000 “psi” number in quotes because it’s wrong, at least when shown with a pounds per square inch label at the end. That 50,000 number is actually an accurate representation of copper units of pressure or CUP. A far less precise way to measure pressure, the method literally relies on looking at how much little copper disks compress when you fire the gun. While there isn’t a consistent mathematical formula that equates CUP to pounds per square inch (PSI) across the board, the difference in this specific case is somewhere around 8,000. In other words, the maximum pressure for 7.62x51mm NATO is about 58,000 psi – not all that far from the 62,000 figure for commercial .308 Winchester.

As both loads are routinely proof tested at far higher levels, this 4,000 isn't a difference that's going to alter the trajectory of Michael Moore's daily Krispy Kreme run.

7.62 NATO vs .308 Win – Case thickness

Measuring the thickness of cartridge cases is kind of a pain, especially since I tend to mash them all up when trying to cut them in half with my Dremel tool. So, I took the shortcut to illustrate the difference. From my big bucket of .308 / 7.62 brass, I selected some representative samples of both commercial .308 Winchester and 7.62x51mm NATO brass and weighed them. I picked several of each and averaged the weights. I didn’t measure the cases because they’ve been fired, so that won’t tell me much other than the general size of the chamber from which they went bang.

  • Hornady .308: 169.6 grains
  • Winchester .308: 163.3 grains
  • Lake City 7.62x51mm NATO: 183.5 grains

That’s a significant difference! Thicker walls combined with similar exterior dimensions means less powder capacity and a lower “top end” and all else the same, lower pressure and velocity.

The thicker brass of 7.62 cases is a significant factor for the reason we'll discuss next.

7.62 NATO vs .308 Winchester – Headspace

Last but not least we get to the real difference. Military rifles for 7.62x51mm NATO can, and usually do, have longer chambers. In things like machine guns powered by ammo made all over the world, there’s got to be some slack for reliable feeding and operation with all that violence going on during the feeding and ejection process. The solution is to make the chamber headspace a bit longer. If you’re not familiar with headspace, think of it as the distance from the bolt face to the point in the chamber that stops forward motion of the cartridge case. If chamber headspace is too long for a cartridge, it will float back and forth in the chamber. If headspace is too small, the bolt will not close properly or will require excess force to close.

How much different is the headspace? The .308 Winchester chamber headspace is between 1.630 and 1.6340 inches (SAAMI Info). The 7.62x51mm NATO is between 1.6355 and 1.6405 inches. While the published numbers show about six-thousandths of an inch difference, it’s not unusual for the headspace in a surplus 7.62 rifle to be 10 or even 15 thousandths longer than that of a commercial .308. That doesn’t sound like a big deal, right up to the point where you fire thinner commercial brass in a long-chambered rifle. The brass will stretch, possibly enough to contribute to a dangerous case rupture. Doing the same thing with thicker military brass is no big deal and the way the system was designed. Thicker brass can handle some extra stretching into a longer chamber throat, so it's no big deal.

The solution to the question is to know your rifle and what its headspace really is. Only then will you know if it is safe to shoot commercial .308 Winchester ammo in a 7.62 chamber.

Figuring out if your headspace is safe is a fairly straightforward deal. You can order a set of .308 Winchester Go / No-Go headspace gauges. After removing extractors and/or ejectors as appropriate in order to remove all sources of tension, 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. A third type, a Field Gauge checks the maximum published chamber size. With some 7.62 rifles, you might find that the bolt closes on the No-Go gauge. As long as the bolt won't close on the Field gauge, you're still within maximum published limits.

The net-net-net

Technically speaking, in terms of specifications, there are differences, but mainly in the chambers of rifles designed to fire each cartridge. 7.62 brass is a bit thicker, and commercial .308 is sometimes loaded to slightly higher pressure, but other than that, the cartridges themselves are pretty much the same.

If you want to be ultra-safe and conservative, fire only 7.62x51m NATO in 7.62 chambered rifles and .308 Winchester in .308 rifles.

Next on the “risk” spectrum is the scenarios of using 7.62x51mm NATO ammo in a .308 chamber. In theory, you might run across ammo that’s particularly long. Ammo might not chamber at all or might require undue pressure to the chamber. That could result in dangerously high pressure. In reality, that would be really unusual. While 7.62 ammo could be significantly longer, that's a pretty rare thing, at least to a significant level, so most people don’t consider it a big deal to use 7.62 ammo in a .308 chambered rifle.

Where you need to be careful is using .308 Winchester commercial ammo in a 7.62x51mm NATO chambered rifle.

While most modern 7.62 chambers are probably fine as they tend to be cut closer to .308 dimensions, it’s always safest to know exactly what you have in terms of headspace. If your rifle has long headspace, stick to 7.62 NATO ammo – don’t use commercial .308.


Tom McHaleAbout

Tom McHale is the author of the Insanely Practical Guides book series that guides new and experienced shooters alike in a fun, approachable, and practical way. His books are available in print and eBook format on Amazon. You can also find him on Google+, Facebook, Twitter and Pinterest.

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ottorod
ottorod
30 days ago

Excellent article. Thanks for taking the time to explain this in detail. I take from the article that the most critical difference is the rifle headspace. I have a Scar 17S, and I wonder if it would be absolutely the same to fire 7.62x51mm or .308 taking into account the Scar 17S headspace. Thanks.

keithnh
keithnh
2 months ago

Bought my M1A from a gun-shop in NH. Salesman was an enthusiast who had an issue with the rifle that permanently scarred his face. This issue may have been part of it. I had never really understood what had happened to him or how to take steps to avoid it. Had seen the 14’s on armory detail in Parris Island when I was carrying a 16, had always liked the appearance, weight and once I owned a civilian model the way it feels and fires. Nice article I will have to see what my rifle is actually chambered for now,… Read more »

Ej harbet
Ej harbet
3 months ago

Beginning of the article had me thinking I need to get my rot eggs that I’m saving for the next antifa suppression strike.I put em away.very good article! Kind of makes me thing of garand when he designed the m1 for the. 276 petterson round only to be told by some Indian war Era remf that they wanted it in 06 because of all the ww1 leftovers.

Bill
Bill
3 months ago

I have long wondered about the so-called “catastrophic failure” claim given to incipient stretching, and ultimately separation, of the side walls of a cartridge case due to excess head space. I have noticed that, in some cases, high power rifle cartridges such as 30-06 have been (and sometimes still are) made with a plastic body separating a brass head and a brass neck. The only really strong part of a case is the head, which seals the breech, while the rest of the case is pretty much just a weak powder container. I have fired some old WWII cartridges that… Read more »

pigpen51
pigpen51
3 months ago

This is one of the most helpful articles to me that describes both the practical theory of the .308 v 7.62 NATO rounds, plus the meaning of headspace and how it relates to guns and ammunition. I had never really understood exactly what headspace was and how to measure it, or why it was so important. Now, I can both understand it and apply it to my own guns. I have a 1916 Spanish Mauser that has been converted to .308 Winchester, purchased from Classic Firearms some time ago. I have finished fixing it up cosmetically and have installed a… Read more »

JayWPB
JayWPB
3 months ago

$973.90 for The paperback edition of The Rookie’s Guide to Guns and Shooting? I don’t think I’ll be buying that one.

Heed the Call-up
Heed the Call-up
3 months ago
Reply to  JayWPB

Jay, not sure where you saw that price, but the link in the “About Tom McHale” has a list price of $9.95.

Mike11C
Mike11C
3 months ago

When I got my Remington 700 5R, chambered in .308, a few years ago, I was getting ready to buy some ammo. I found some 175 grain .308 Federal Gold Medal Match at a good price and, I bought some. Shortly afterward, I found some 175 grain 7.62×51 Federal Gold Medal Match ammo on sale. According to the information on the boxes, both had the same specs. So, I emailed Federal and asked them what was different about them. They responded that the 7.62 cases were not polished, like the .308, because military specs require the cases to be “visibly… Read more »

Bill
Bill
3 months ago
Reply to  Mike11C

When using ammunition hand loaded to higher .308 performance specs, I find that there is somewhat more violent cycling in some .308 semi-autos, with cases not infrequently blowing out their primers and the extractor leaving a deep dent in the case rim. This harsh activity can be tamed somewhat by fine-tuning the firearm, but a less strongly charged 7.62×51 load more immediately cycles the action in a comfortable way and is less damaging to the brass. Therefore, when using a semi-auto .308, I find it most practical to stick with slightly lighter charges, more at the power of a 7.62×51… Read more »

JPM
JPM
3 months ago

The logic, and science behind the non-interchangeability of .223 Remington and 5.56 NATO is profound and based on experience and extensive testing over many years indicating a viable concern, including actual catastrophic failures due solely to the lack of interchangeability. The claim that the same applies to .308 Winchester vs. 7.62 NATO is speculative and theoretical at best. We have more examples over a far greater period of time of the successful interchanging between .308 with 7.62, and vice versa, and are continuing to do so, without any dangerous or injurious results, than we have had with .223 vs 5.56… Read more »

Hankus
Hankus
3 months ago

An important aspect that you didn’t touch on is handloading. I would hope that anyone who “rolls their own” would be savvy to the difference in brass, and sorts their brass carefully. Throw your favorite hot .308 powder charge into a 7.62 case and you’re cruising for a rude surprise.

Knute
Knute
3 months ago
Reply to  Hankus

But not if you work up to that hot load in whatever case you choose. Military or commercial. Taking a max load and switching out to a different case (even if both are commercial cases, but just different brand names) is just asking for trouble. The only component that one might be able to swap out is the primers, but I work up again even at that. For semi autos, I just stay under max and do them in mass on a progressive press. If I want max performance out of one then I go back to working up loads… Read more »

Vince
Vince
3 months ago

Your article begs the question, is it possible to design to accept either cartridge, same as done with .223/5.56?

Bill
Bill
3 months ago
Reply to  Vince

Yes. A rifle rated to use .308 Win is typically considered fine to use with 7.62×51. This rule of thumb is extremely common, and most .308 Win rifle manufacturers say as much. I would simply be a little more cautious if using a rifle made for 7.62×51 but not for .308 Win. This is very similar to the .223/5.56 issue, but reversed, in that, with .308 Win, it is the commercial load, not the military load, that has the higher pressure.

Rock
Rock
3 months ago

Soooo, if it is a relatively NEW manufactured weapon it should be safe to say that it has been and is more than likely chambered for use with both the 7.62 NATO and the .308 Winchester. Battlefield “pickups” or old surplus ammo should be checked and verified before mixing the ammo. Right ?

Tryagain
Tryagain
1 year ago

Why didnt you buy some reloading brass and measure them? And then load them withithout powder to proper cartridge specs for length by the squisher, and then drill out the fake bullet used?

D.million
D.million
1 year ago

Interesting and informative. Thanks!

Stevan Kaighen
Stevan Kaighen
1 year ago

Tom McHale
I doubt you will see this but I wanted to thank you for this article. The information you provided and the dialogue you provoked was extremely informative. Best wishes and thank you. Stevan