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« Firing In Eights | Main | Ballistics and cartridges »

March 21, 2004

A Tale of Two Cartridges

Despite popular belief the .308 Winchester & 7.62x51mm NATO chamberings are different.

In my youth I never paid it much attention when someone would say that he picked up some .308 ammo for his FN/FAL or his M1A. It's not that I didn't care; I simply didn't know that there was an interchangeability issue.

Part of the reason for my ignorance was a matter of taste: I never had any interest in owning a rifle in either chambering. Now before y'all get the wrong impression I don't think there's anything wrong with the .308 Winchester cartridge, or the 7.62x51mm NATO cartridge. Like so many other cartridges they are fine as long as you don't try to make them do things they weren't designed for. Deer hunting, combat & target shooting are fine uses for the aforementioned cartridges. I wouldn't want to face down a charging Cape Buffalo with one though, or hunt squirrels without using a seriously reduced load.

But I've always been partial to the .30-06 Springfield. For my purposes I have no need for a .30-06 and a .308 Winchester or 7.62x51mm NATO. I'll let y'all in on another secret - I don't care for Winchester products. Winchester makes some great stuff & I don't doubt for a minute that many people are exceedingly well served by their Winchesters. I just ain't partial to them. There's no particular reason: it's just a matter of preference.

But in any case what I like & what I prefer to use shouldn't be seen as a negative testimonial to either the cartridges or company mentioned above. Some people don't like Mozart, others don't like Prince. & that's cool as long as we don't confuse not liking a product with that product being of poor quality. After all you can't blame someone if they don't like Mozart or Prince but you can blame them if they attempt to justify their dislike by accusing either gentleman of lacking talent.

All of that is simply to stress that I have nothing against the .308 Winchester or 7.62x51mm NATO chamberings except that I prefer the .30-06 Springfield & don't have a need for 2 different .308 bore rifles in my life at the moment.

That's why until a few years back I just assumed like everyone else that the .308 Winchester was just the commercial version of the 7.62x51mm NATO cartridge. I was wrong.

xwing.jpg

The 7.62x51mm NATO cartridge started out as the experimental T65 cartridge. This should not be confused with the Incom T65 which is a much older & completely different military endeavor. The T65 was basically a lengthened version of the .300 Savage case & was a joint effort between the U.S. military & Winchester.

The whole purpose behind the T65 project (the cartridge - not the starfighter) was to produce a cartridge that was compatible with the military's ideas on a new rifle. The Godless Heathens military planners wanted a select fire rifle that could fill the role of an assault weapon & a battle rifle. The .30-06 Springfield cartridge was to be replaced because the Blasphemers & Usurpers military planners felt that it was too powerful for use in a select fire weapon (in other words it made the weapon barely controllable) & that each .30-06 cartridge weighed too much. The T65 was viewed as a more controllable, lighter weight substitute for the .30-06. Because it was less powerful it could be used in select fire weapons with greater ease, & its lighter weight per round meant that a soldier could either carry more ammo for the same weight or the same number of rounds for less weight.

The T65 if you haven't guessed became the 7.62x51mm NATO cartridge (actually the T65E3 was the final version before it was adopted if I recall). But Winchester introduced their version of the T65 in 1952 as the .308 Winchester. It was later on (1953 or 1854 I believe) that the military finalized the cartridge as the 7.62x51mm NATO. It wasn't until 1957 that the cartridge actually had a platform in the U.S. Rifle, Cal. 7.62 M14.

Now that you have the basic history let's move on to the complicated stuff:

Headspace is term for the dimensions of a firearm's chamber & a cartridge. Most people assume headspace only refers to the internal dimensions of a chamber, but this is only half correct. Think of it as being similar to a shoe size measurement: size 10 gives you an idea if your foot will fit into a shoe or not. But the measurement itself is simply a way of determining if the cavity is too big, too small, or just right for your foot to fit into. Knowing that a shoe is a size 10 will do you no good unless you know that your foot is a size ten as well. The measurement is commonly used in reference to one object, but it's purpose is to determine compatibility of two objects.

So just as there is a headspace measurement for the chamber of a firearm there's a headspace measurement for the cartridge.

Standards for headspace in the U.S. are set by SAAMI - the Sporting Arms & Ammunition Manufacturers' Institute. (Here's the Yahoo cache of their homepage as it seems to be down at the moment.)

Now the headspace of a chamber is typically measured with 3 gauges: a Go gauge, a No Go gauge & a Field Reject gauge. These gauges are all cylinders of varying lengths that are made to fit inside the chamber in the place of a cartridge. They measure the distance from the breech face or bolt face to a measuring point within the chamber. This point will vary from cartridge type to cartridge type & sometimes within the same cartridge types. A rimmed cartridge stops on a cut for the rim in the chamber & this is where a gauge for that chamber will be set. A belted cartridge headspaces on the belt cut & its gauge will do so as well. A rimless cartridge will headspace on the case mouth (where the cartridge stops & the bullet begins) & the gauge for such a chamber will be made appropriately. A rimless bottleneck cartridge will headspace on a datum line somewhere forward of the shoulder so a gauge for it will do likewise.

The Go gauge is set at the minimum of the headspace range for that chamber. if a bolt will not close on a Go gauge then there's insufficient headspace which means the cartridge will not fit in the chamber. If that occurs then you have to ream out the chamber until the Go Gauge will fit. If the bolt will close on a Go gauge then the next gauge in the set is used.

A No Go gauge is at a point in between the minimum & maximum acceptable headspace, but usually closer to the maximum. If a bolt won't close on a No Go gauge then your checking is done. The headspace is well below the maximum & above the minimum. If a bolt will close on a No Go gauge then that means the headspace is getting on the long side. Decreased brass life & deteriorating accuracy are the main problems you'll see, although there is an increased risk of case head seperations &/or misfires because of the greater headspace. But the rifle should still be safe to fire as long as the bolt won't close on the next gauge.

A Field Reject gauge marks the maximum limit for headspace. If a bolt will close on a Field Reject gauge the headspace is beyond the maximum allowable limit & the firearm is unsafe to use.

A cartridge headspace gauge usually consists of a cylinder that approximates a minimum headspace chamber. There'll be a step on the outside of the gauge that will mark the maximum cartridge protrusion from the base. If a cartridge extends beyond that, then it is too long & will not fit correctly (if at all) into a nominally headspaced chamber.

Now that you have a basic understanding of headspace gauges, let's look at some numbers:

7.62x51mm NATO Go 1.6355"
7.62x51mm NATO Field Reject 1.6455"

.308 Winchester Go 1.630"
.308 Winchester No Go 1.634"
.308 Winchester Field Reject 1.638"

7.62x51mm NATO minimum cartridge headspace 1.630
7.62x51mm NATO maximum cartridge headspace 1.633
7.62x51mm NATO average cartridge headspace 1.6315

.308 Winchester minimum cartridge headspace 1.627
.308 Winchester maximum cartridge headspace 1.633
.308 Winchester average cartridge headspace 1.630

As you can see there's a notable difference between the headspace of the .308 Winchester & the 7.62x51mm NATO chambers. To put it in a more direct contrast:

7.62x51mm NATO Go 1.6355"
.308 Winchester Go 1.630"

That's a difference of .0055"

7.62x51mm NATO Field Reject 1.6455"
.308 Winchester Field Reject 1.638"

That's a difference of .0075"

Now you might be thinking that .0075" is no big deal & certainly not .0055". Here's why it is a big deal:

The headspace tolerance for the .308 Winchester is .008" from minimum to maximum. SAAMI feels that if your chamber is .008" over the minimum then you'll have too much headspace to safely fire a cartridge. The problems of excessive headspace vary with the severity of the excess from reduced accuracy & short case life to case head separations & even self disassembly (a polite way of saying that you'll blow your firearm up). .008" is deemed to be too much of a safety risk & any gunsmith that finds that much headspace in a .308 Winchester will tell you not to fire your rifle until the headspace is corrected. But keep in mind this is just the difference in chamber headspace between the two.

Where the real problem lies is in the cartridge headspace tolerances:

7.62x51mm NATO minimum cartridge headspace 1.630"
.308 Winchester minimum cartridge headspace 1.627"

7.62x51mm NATO maximum cartridge headspace 1.633"
.308 Winchester maximum cartridge headspace 1.633"

7.62x51mm NATO average cartridge headspace 1.6315"
.308 Winchester average cartridge headspace 1.630"

If you'll note the maximum for both cartridges is the same - 1.633". The minimum varies by .003 & the average by .0015".

Now let's suppose we have a 7.62x51mm NATO chamber that has a headspace of 1.6450". If we chamber a 7.62x51mm NATO cartridge with an average headspace then we'll see a space of .0135" that the catridge case will have to fill inside the chamber. That's .0035" more than the allowable difference between minimum & maximum chamber headspace in the 7.62x51mm NATO chamber.

Now why would the military allow for such tolerances if they're unsafe? Because military brass is thicker than commercial brass. (The thickness is added inside the case & is why you have to reduce the powder charge when you reload military brass - it has less internal space because its walls are thicker.) The thicker brass is more capable of stretching without rupturing than commercial brass. & another reason why this is safe with the military is that military brass is used once. The military doesn't reload its cases so it doesn't have to take work hardened brass into consideration. (When you reload ammunition the brass is resized. This causes a loss of elasticity in the brass & with repeated firings & reloadings the brass will rupture instead of stretching to fit the chamber.) Thicker brass that's only used once can tolerate the extra .0135" of space that would be unsafe with commercial brass.

Now let's say we put some commercial ammo in our 1.645" chamber. Further let's say that the commercial brass measures at 1.631" (average cartridge headspace + .001"). That's .014" of space for the case to move inside the chamber. & that's with thinner walled brass that may have been work hardened. Now will that combination result in a case head separation? Eventually yes. .014" is a lot of space when you're talking about how much a case has to stretch. But it's entirely possible to fire the cartridge without having anything bad happen. It is not however a risk that I'd be willing to take & I wouldn't expect anyone else to take it for me.

Now let's look at the other side of the coin - insufficient headspace:

Let's say we have a brand new .308 Winchester rifle with a minimum headspace chamber. It measures at 1.630". We find a deal on some Mil-Surp 7.62x51mm NATO ammo & go to the range. Ammo that averages at 1.6315" will be a very, very tight fit in a 1.630" chambered rifle. Too tight in fact. If you are able to chamber the cartridge you will have higher than normal pressures. Given a few other variables this could result in redlining the chamber pressure & having the rifle self disassemble a few inches from your face. If it's an autoloader then there's a real good chance you'll have a slam fire (where the cartridge fires as the bolt lock up without any desire for it to do so on your part) or an out of battery fire (where the cartridge fires before the bolt is locked) which will cause a self disassembling firearm to materialize in your hands. .0015" is a very big deal when you lack that much space.

Of course the above scenario could happen with .308 Winchester ammo that's larger than average, but it's not as likely as encountering 7.62x51mm NATO ammo that would create insufficient headspace.

So the two cartridges should not be automatically considered interchangeable even though the cartridges themselves are very close to each other. The headspace difference of the rifle's they are chambered for is what makes some combinations dangerous.

Now you will no doubt here people tell you that they have a firearm chambered in one & they've been using ammo marked as the other for years without a problem. What can I say? There are also people that go for 9 months between oil changes & their car doesn't break down. That doesn't mean you should follow their example.

Firearms are designed with a safety margin. They won't instantly blow up the second you're 1 PSI over the maximum chamber pressure. But with repeated abuse they will fail. Odds are the people that are using the cartridges interchangeably are doing so safely without realizing why.

If their 7.62x51mm NATO chamber is towards the minimum safe headspace (1.6355") then a .308 Winchester cartridge that measures 1.630" will result in .0055" of free space that the cartridge must stretch to fill.
For comparison a .308 Winchester chamber that measures 1.6355" (No Go dimension -.0005") will leave .0055" of room for a 1.630" cartridge. It'll still be safe to fire but will start to show a decrease in accuracy as well as a shortening of brass life.

Now the thing about chamber headspace is that it isn't a permanent dimension. Over time headspace will increase as the chamber is worn through normal use. After a few thousand rounds that 7.62x51mm NATO chamber that was at 1.6355" may very well be at 1.6375" which would be pushing what SAAMI considers the maximum allowable room between a cartridge & a chamber in .308 Winchester. When it gets to that point you're looking at .0075" of space that the cartridge must stretch to fill & you will notice a decrease in brass life.

Similarly a person may have a .308 Winchester & use 7.62x51mm NATO Mil-Surp ammo with no problems. If it's a bolt action then the chamber is right at or over the working average for the cartridge - specifically his chamber will be 1.6315" or longer. If it's a semi-auto then I'd say his chamber must be at 1.632" or longer for everything to work fine. If his chamber is any shorter than that for either type of action then he's either getting some ammo that's shorter than average or hes just real lucky.

Another thing I've heard is that the cartridge manufacturers & reloading manuals don't differentiate between the two cartridges so they must be the same. This is incorrect - at least partially. Commercial cartridge manufacturers make the .308 Winchester cartridge. They also make the .40 S&W cartridge. In neither case do they specifically state you shouldn't use .308 Winchester in a 7.62x51mm rifle or .40 S&W cartridges in a 10mm pistol. What they do say is that you should only use the specific cartridge in firearms marked for that identical cartridge. "Only use ammunition that exactly matches the markings on your gun" is generally how it's phrased. Now if you have a box that says ".308 Winchester" you can hold it up to the barrel of your military surplus rifle & see if it matches the "7.62x51mm NATO" marking on the barrel. Go on - give it a try. I'll wait.

So cartridge makers do warn you about using the incorrect ammo; you just have to read the box.

As for the reloading industry - it's a misunderstanding to infer anything from them. The differences between the cartridges are in headspace - not loading data. The .41 AE uses the same loading recipes as the .40 S&W yet no one would think that the two cartridges are interchangeable. As far as reloading dies go they only affect the brass. A resizing die takes brass & reshapes it to a slightly smaller dimension than it is after it's been fired. This does not mean the brass has to be identical to a cartridge for it to be effective. It just has to be close. You can use .30-06 brass to make cartridges in .270 Winchester, .35 Whelen & .30-06 Ackley Improved. With the exception of the .30-06 A.I. they're not interchangeable & even with the .30-06 A.I. chamber full power '06 loads aren't recommended as a steady diet.

7.62x51mm NATO brass is close enough to be made into .308 Winchester brass. & because they are so close in cartridge headspace there's no need to differentiate between the two in regards to reloading equipment. After all the main difference between the two cartridges isn't the cartridge headspace but the chamber headspace. Reloading manuals & equipment just doesn't deal with chamber headspace. So to say that because the reloading industry doesn't point out the difference in the cartridges is to misunderstand what reloading is about.

From what I understand gunsmiths (at least the few I anecdotally know about) have been reaming .308 Winchester chambers to 1.632" (.308 Winchester minimum headspace + .002") unless the customer specifically requests a Match chamber (which will have tighter dimensions). This eliminates all concerns about using 7.62x51mm NATO ammo in a .308 Winchester chamber. I don't know any gunsmiths who ream 7.62x51mm NATO chambers but I've heard of a few who mark their 1.632" barrels as such. Again this eliminates any concerns.

Now I am not saying it is absolutely impossible to use a 7.62x51mm NATO cartridge in a .308 Winchester chamber or vice versa. What I am saying is that you should know in detail what you're doing instead of relying on talk you heard at the gunshop as you're paying 10% over MSRP while thinking you got a deal cause it was on "sale". If you have a .308 Winchester or 7.62x51mm NATO rifle & want to interchange ammo then go to a gunsmith whom you trust & have him measure the chamber headspace. Don't just rely on the gauges: get him to give you a number in inches. Then you can make a knowledgeable decision.

& don't think for a second that you won't hear from otherwise educated people that there is no difference between the two cartridges. I doubt the truth will become popular in this century. Just remember that there is a difference & that as long as you're aware of the specifics of your rifle you can decide for yourself if you can interchange the two cartridges. Most important is that whether you can or can't interchange them you'll understand why.

For some related reading I'll leave you with the following:

7.62x51mm NATO or 308 Winchester? An Armourer Explains the Differences

Debunking the myth of "NATO standard" ammunition

What's the Difference between .308 Winchester & 7.62x51mm NATO?

Tempest in a Cartridge Case: No, Virginia, .308 Win. & 7.62 NATO Are Not Identical

.308 Winchester history

I feel it's appropriate to give a brief introduction to this next link & in the process offer an explanation for why a dedicated '06 guy took an interest in two cartridges that he has no immediate interest in owning:

You might have noticed but I have a weakness obsession fondness for the M1 Garand. A few years back I purchased a book that deals with the mechanics of the Garand as well as the M14/M1A. In it I found out that the 7.62x51mm NATO & .308 Winchester were not identical. I casually mentioned this on a message forum I participated in & as a result of the ensuing knock down drag out blood feud argument discussion for the next few days I immersed myself in learning all I could about headspace in order to explain why these cartridges were different. The book that started it all for me was:

"The U.S. .30 Caliber Gas Operated Service Rifles: A Shop Manual Volumes 1 & 2" by Jerry Kuhnhausen

It sells for just under $50 & is invaluable to anyone who owns an M1 Garand or an M14/M1A. There are few books on firearms I could recommend more strongly.


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