Tom McHale writes [via 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

Doing 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. 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 its 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.

——–

About

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

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

  1. wouldn’t it have been easier to just say, “it’s just like the 5.56 being chambered in a .223 chamber, but bigger, unsafe, don’t do it, other way around is ok”? ?

    • He could. But to be believable, you have to provide proof. Only in forums and post replies like this do people make bold statements of supposed fact and expect the readers to believe the information just because he posted it.

  2. “dangerous case rupture.”

    Can someone please explain why it’s dangerous when a case ruptures? I have seen it happen. Friend was running .44 Magnum in a .45 Colt Henry rifle. After about three shots he discovered his error. No ill effects except a profound lack of accuracy. I’ve also seen ruptured pistol cases (probably ones that were reloaded multiple times).

    My understanding is that when the primer is struck, the case is little more than a gasket to keep gas from escaping from the breach. As long as the pressure doesn’t blow up the gun, what’s the danger?

    • “Can someone please explain why it’s dangerous when a case ruptures?”

      How close is your face to the chamber when you fire the rifle?

      Do you you think exposing your face to 60K PSI of hot gas is wise?

      • To be fair, as long as your bolt is properly closed, the gas flow will be sonically choked. Basically, no gas will pass the seal at a velocity greater than the local speed of sound. (Which can vary drastically during firing, but that’s a separate discussion.) Are you going to enjoy the experience? Probably not. Is it going to be a major issue in a rifle where the chamber and bolt are fully enclosed? (Like an AR10 or G3) Not really. By the time the gas escapes the receiver it will have expanded drastically and thus cooled significantly. (Remember, your total gas flow rate into the receiver is sonically choked by the bolt-chamber interface.) Basically, a case rupture in a weapon where the chamber is not exposed is a minor inconvenience. Now if you’re talking about a rifle with an open bolt-chamber interface like an M1 or M14… Yeah… it could get unplesant.

    • if you are shooting a modern bolt action rifle, than not a whole lot. the rifle is designed to vent gasses from a case rupture away from the shooter’s face. the main problem comes with a second round. if the ruptured case lost enough pressure to stop the bullet in the barrel you could get a barrel rupture and that is a problem. self loaders are a little different. gas venting is ok, but the action may not cycle (with the possibility of a stuck bullet) or (depending how the case ruptured) tear off the back of the case and disable the rifle.

    • In your friend’s situation, the rupture occurred because the brass was the wrong caliber. Pressures were presumably not high because the .44 caliber bullet in a .45 caliber barrel would not have any kind of proper gas seal. In a case of the proper caliber, a rupture allows hot gas at extremely high pressure (due to the bullet sealing the bore properly) to escape (brass is designed as a gas containment vessel, since it stretches). This can lead to everything from erosion of the chamber/bolt, to getting a facefull of hot expanding gases and/or brass slivers. Not a good thing.

      • Ok, but we’re not talking about an unsupported case (such as with a Glock) rupturing at the base.

        This article mentions the possibility of a case sidewall (I presume) rupturing because it’s thin and stretches too much as it lengthens to fill the oversized 7.62 NATO chamber. The base of the case is still acting as a gasket between the chamber and the bolt, and I presume it would still do its job of containing the pressure.

  3. Yes an interesting explanation. I always thought they were identical. That’s why I come to TTAG…

  4. I have read about some surplus M-1s being .308, rather than .30-06. Does that mean the rifle chamber is larger than OEM due to usage, or that both the barrel and chamber have been retro-fitted into the stock such that the .308 will fit?

    Or, none of the above because I have no clues regarding military rifles?

    • Not sure where you got that little tidbit. 30-06 and .308 are drastically different cartridges. Similar terminal effects in military loadings, but 30-06 will never fit in a .308 chamber. I would be sure not to be in the room is somebody was stupid enough to try.

    • I read an article (in the NRA magazine IIRC) about Navy M1 Garands that have had 7.62×51 head inserts installed in the chambers. They essentially shorten the shoulder distance to allow chambering of the newer, shorter cartridge.

    • Sam. For a brief time Springfield, the private company not the .gov arsenal, manufactured a modern m1 rifle chambered in .308. The bore diameter is identical between .308 and .30-06.

      I don’t know how many were made or sold but they’re out there.

    • The .308 is a shortened version of the 30-06, with about a half inch shorter case. It’s slightly weaker in hunting loads, but about the same in military loads, as Serge mentioned. The .308 Garands were a later variant made by the Navy. The Italians also made a Garand clone in 7.62 Nato, the BM 59.

        • Beretta took existing m1s and made them into what were basically m14s. Some, very few if I recall, were sold surplus here. They had a good rep. But like the m1 and the m14 they were heavy.

        • “But like the m1 and the m14 they were heavy.”

          Thought about that today. Looking back at the beginning, those rifles and muskets were really heavy. then we got to the 1903 Springfield at about 9lbs. That was probably considered pretty light compared to the Civil War rifles. Now, we seem to be spoiled to the idea of a SAW that weighs 4.lb with a belt of ammo weighing only another 6oz. Not saying lightweight is good or bad, just how our perceptions compare to the eras that got us here.

        • Sam. I carried a 7 pound m16. But the rest of the load makes up the difference in the weight of the weapon.

          The grunt is always overloaded. Always tired. Hot. Hungry. Thirsty. It’s a hell of a weight loss program.

      • The .308 is not actually a shortened .30-06 but based on the .300 Sav age ca se. The differences are minimal, but for instance, the distance from the bottom of the rim to the edge of the base is a bit longer on the .308 and the .308 has less taper to the shoulder than .30-06. Probably hard to tell just by looking at them though.

    • Around 15 years ago there was a guy competing at the Nationals at Camp Perry with a Garand in .308, sorry, I don’t know whether he personally altered it or not. I would have loved a closer look just to see how the feed worked.

      • If you want a .308 Garand, all you have to do is replace the barrel. It is customary to install an aluminum or plastic block in the front of the magazine so that a clip loaded with .30-06 cannot be inserted into the magazine, but it is my understanding that it is not required.

        If you don’t want a heavy match barrel, Criterion makes an excellent GI contour .308 Garand barrel. The mag spacers are easy to find as well.

        • Just received an email from CMP. They have .308 M-1s on the shelf, ready to sell.

        • The price they have listed on their website is pretty fair. I’d probably want to go to their store and pick one out since they advertise that the receiver may have some pitting.

  5. Fortunately I get good results with NATO ammo in my AR10 and that’s what I stick with. My .308 rifles see commercial ammo and occasionally a NATO round. I’ve see a variety of OAL difference between the commercial ammo due to different bullet designs. For this reason I stick to NATO ammo in the evil black gun.
    I had feeding problems on my savage 11 with some 180gr Hornady rounds that were really long. I use 150 and 168 grain loads and I’m quite happy with the results.

    • I was gifted a box of 190 grain .308s. They caused feeding problems in several rifles. IMHO if you need heavier loads than your 168 in .308 you need a .30-06 or even .300 win mag.

  6. If you want to start a good bar fight, ….

    I’d rather start a good cat fight. Any ideas there?

  7. OK, I’m going to make this quick, as I have somewhere to be this evening.

    Case head separations generally don’t happen in new (ie, never before fired brass case ammo) when there’s only 5 to 6 thousandths too much headspace. Case head separations generally involve one or two things:

    1. The brass has stretched, and thinned out in an annular ring around the brass, just about he point where the web inside the case starts to thicken. Why has the brass stretched to a point where you see this annular ring of bright brass just forward of where the web starts? Because you’re shooting your brass in a chamber that has excessive headspace, and you keep re-sizing it with full length sizing dies. This would likely be a semi-auto rifle. In a bolt gun, you don’t need to full-length size on every reloading. If you have a chamber in a bolt gun that’s a bit too long, then you fire your brass the first time, and the brass stretches, and… now you should only neck-size and trim the brass. That’s it, you’ve sized the brass for that one rifle with the rifle’s chamber.

    But if you’re feeding the brass into a semi-auto rifle, you will typically full-length size it so it feeds reliably into the chamber.

    2. Your chamber headspace is wildly out of spec – ie, it not only fails a no-go gage, it fails a “field” gage.

    Let me explain quickly the three headspacing gages out there: There’s a “go” gage, a “no-go” and “field” gage we gunsmiths use to determine what’s going to happen when we put a cartridge into this chamber and pull the trigger.

    In all cases, the gage is inserted into a clean chamber (and the gage better be clean too). I mean the chamber better be really clean – no grit, no brass dust/wipes, no powder fouling, no nothing. We “strip” the bolt of springs, ejectors and extractors.

    Now we put in a gage and we gently close the bolt on the gage. In a bolt gun, I use a stripped bolt and allow the bolt to drop closed under the weight of the handle alone. I do not ever came the bolt down with my hand, and that’s a good way to mess up the chamber or the gage.

    When the bolt closes on a “go” gage, this means that we have a chamber big enough to accept a spec cartridge. “Go” gages are used typically only when putting a new barrel on a receiver – we need to make sure the chamber is “deep” enough to accept the full length of a cartridge. We want the bolt to close on a “go” gage.

    We pull out the “go” gage and then put in a “no-go” gage. Now the bolt should NOT close. Typically, a “no go” gage is 0.004 to 0.006 (depending on the cartridge in question) longer than the “go” gage. For a new chamber or a new rifle, a bolt that closes on a “no-go” gage should be rejected, and the barrel set back a bit (perhaps 0.004″) to get back to a point where the bolt closes on the “go,” but not on the “no-go.”

    Now, finding an old rifle, especially an old bolt action rifle, where the bolt closes on a “no-go” gage isn’t the end of the world. It means that the rifle is just going to be hard on brass – ie, the brass is going to stretch a bit. If this rifle is going to be mated with the ammo (if you reload) or fed only new ammo (if you don’t reload), then you might had no issues.

    Lastly, there’s the “field” gage. A rifle that closes on a “field” gage should be rejected for use. A “field” gage bolt closure is where you absolutely reject the gun for firing live ammo until the barrel is set back or a new barrel w/ a new chamber is hung on the action, because many field gages are 0.010 to 0.012 longer than “0 headspace.)

    Lastly, should a rifle ever have “zero headspace?” Yes, but only if you’re willing to deal with what comes with “zero headspace” – namely, you must keep that chamber clean, clean, clean in order to avoid cramming a cartridge in there that then causes neck crimping and higher pressures when you light it off. Benchrest shooters might have a chamber that is at “zero headspace,” but if done in a AR or other type of semi-auto rifle, you might end up with situations where the cartridge is chambered, but the bolt is every-so-slightly out of battery – this can be a very dangerous situation for a semi-auto shooter to be in. In a semi-auto, you don’t want to be in a situation where the bolt just barely closes on the “go” gage – you want the bolt to close easily on the “go” gage, but not close on the “no-go” gage. Benchrest folks sometimes want their bolt guns to just barely close on the “go” gage, meaning the chamber has minimum headspace.

  8. Excellent article, with this one misstatement:
    “Thicker brass can handle some extra stretching into a longer chamber throat”
    Thicker brass can stretch more, but the chamber “throat”, is the area of the bore that is without rifling lands, in order to give the bullet a ‘running start’ before taking the rifling. The throat has nothing whatsoever to do with headspace in any design firearm I’ve ever seen. If it did, the depth the bullet is seated into the case would change the headspace, which is an ‘accident’ just waiting to happen.

  9. Wasn’t the 7.62 also a way to take the “wasted” space out of the aught six? *

    Do I recall hearing the salt shaker noise when last messed with 30-06.

    *”Hatcher’s Notebook”?

    • You need to read up on the development of the M14 to find the answer. The issue was that the DOD wanted an infantry rifle that fired a “full power .30 cal cartridge” and John Garand himself told the Army that it wasn’t possible in a 10lb rifle.

      And it isn’t. Look at the weight of the BAR.

      Well, the DOD figured if they knocked a few hundred fps off the MV for the .30 cartridge, and they cut down the length a little bit, they could make the ammo a little lighter, cycle the actions a little faster, and get what they wanted.

      They turned out to be wrong.

      • A few hundred fps? Last I checked the velocity difference between .308 and .30-06 in a 150gr. load is 90fps with the 7.62×51 running I think another 50fps slower.

        • The spec for M2 .30-06 ball ammo was about 2805 fps (muzzle) and M80 7.62 ball spec is about 2700 fps.

          Using modern powders, you can probably achieve a very marginal difference between the two.

    • The old military loads for .30-06 are several hundred fps slower than most modern hunting loads. In fact it is easy to duplicate the original performance of the .300 H&H Magnum with a .30-06.

      This is due to the much more advanced powders that we have today. The 1886 Lebel was the first rifle to use smokeless powder. Seventeen years later when we adopted the 1903 Springfield the available powders were still pretty primitive. Fifty years later it wasn’t difficult to duplicate the velocity of the original mil spec .30-06 loads in a case 1/2 inch shorter.

    • The old military loads for .30-06 are several hundred fps slower than most modern hunting loads. In fact it is easy to duplicate the original performance of the .300 H&H Magnum with a .30-06.

      This is due to the much more advanced powders that we have today. The 1886 Lebel was the first rifle to use smokeless powder. Seventeen years later when we adopted the 1903 Springfield the available powders were still pretty primitive. Fifty years later it wasn’t difficult to duplicate the velocity of the original mil spec .30-06 loads in a case 1/2 inch shorter.

  10. I’m not sure if anyone touched on this, I scrolled to the bottom. The BIG issue is when reloading. IF you roll your own .308 ammo to top end pressures, but you are using milsurp 7.62 brass, that thicker wall becomes a CRUCIAL issue.

    You can easily get pressure spikes and have a big bad kaboom when loading 7.62 NATO brass to .308 win specs. The thicker brass means decreased volume meaning quicker pressure increases to maximum and beyond….not a good thing!!!!

    • All true, but the same thing can be said of any/all calibers. When reloading, loads must always be ‘worked up’ to maximum. Every chamber is different. tighter or looser, distance of ‘freebore'(the area of no lands just ahead of the bullet), bullet seating depth/ bullet ogive, all have an effect on pressures. If one just picks the max load and seats the bullet touching the lands, or that rifle has a tighter than usual chamber spec, overpressure can result.
      On the other hand some rifles can take even above max loads in complete safety, IF one works up slowly and knows how to look for pressure signs. If this all seems to be Greek, definitely “do not try this at home”!

  11. Overall this was a pretty good article. As far as I know, US made .308 and 7.62 ammo has the same external dimensions.

    I think the biggest reason to be wary of firing .308 in a 7.62 rifle is because military ammo is loaded with powders that produce a pressure curve compatible with gas operated automatic weapons. You are probably pretty safe with most of the basic hunting loads like Remington 150 grain Core-Lokt. I would avoid anything loaded with heavier bullets or premium ammo, especially if it advertises higher velocities. In these cases there is a good chance the ammo may use slower powders that produce high port pressures that will beat up the rifle. 7.62 is easy to find and easy to reload. Since most 7.62 rifles are kinda pricey these days, I don’t see a reason to beat them up.

  12. Good info, this is similar to trying to use once shot reloaded brass without knowing the head space of the rifle in which the brass was shot, nor knowing your own!

  13. The only real difference you will run across is that the military 7.62×51 cases are a lot thicker than commercial .308, and can’t hold as much powder when handloading. Indeed, it is impossible to fit a max charge of 4895 specified for a commercial case into an LC military case. As for factory loaded rounds, it’s fine to interchange in either direction. Note that this relationship is opposite in the 5.56 vs. .223 scenario. In that one, the military wants to pack as much powder in there as possible, so the LC cases are thinner than commercial, but not by much, and the 5.56 chamber has a longer leade (freebore) than the .223, and really hot .mil rounds can be over pressure in a .223.

  14. As a receiver of gas blowback, I can say without reservation, “it stings a bit” when experienced from a semi auto weapon.

    Mine happened to be a triple loaded Colt 1911 10mm. Though the Colt functioned as it should have by locking up and preventing a full blowback, I did get a face full of gas and powder.

    • Even in worst case scenarios, that is generally the case. A face full of hot gas(that’s why the eye protection…), and perhaps a damaged firearm. The shooter getting severely injured in a firearms failure is so rare as to be virtually non-existent.

  15. Excellent article. The only significant oversight (omission?) is that NATO milspec cases, in addition to other manufacturer marks, are head-stamp marked with a cross sign surrounded by a circle … if that cross/circle isn’t there, it’s not NATO milspec.

  16. If you reload check your primers very closely. You can put a 308 load in a 7.62 and then get higher pressures when fired. If you sort your cases carefully and load accordingly all is OK. If not that 7.62 can lead to a blown primer. This is only when loading hot and near limits. Still I have seen it happen.

  17. The photo at the top of the article is an M-14 I’m guessing which from what I understand should only use 7.62 NATO. Now I own an M1A which I understand should also only use 7.62 NATO. However mine has .308NM stamped on the barrel.This may sound a little dumb but the chamber is part of the barrel, right? The rear end? So my M1A is chambered for .308, correct? I’ve read a lot of forums about M-14’s and M1A’s and what each rifle should and shouldn’t use and there is so much difference of opinion I just wanted to see what you guys think. And besides case thickness and some other things there is discussion of harder primers in the 7.62 ammo. Does that make much of a difference in a civilian M1A?

    Yes I know it’s a Springfield product but I bought it about 5 years ago, before
    Springfield sinned.

    • The only thing you really need to worry about with the M1A is powder burn rate. You should be ok shooting .308 loaded with 147-150 grain fmj bullets. I have fired both PMC and Remington .308 ammo in my M1A, but I almost never buy factory rifle ammo.

      Since the cheapest way to buy ammo is to buy online in bulk, you are probably better off buying 500 rounds of 7.62 at a time.

      If you reload just make sure you full length resize, use a case gauge and avoid Federal primers (they are the most sensitive of the US primers).

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