The Truth About 7.62x51mm NATO and 308 Winchester


#1

I have found this interesting article on the web. It says that despite of the ubicuous misinformations about case thickness and pressures, the 7.62 mm Nato round is almost identical to the .308 Winchester cartridge, and fully interchangeable. In my humble opinion, that’s the truth.

The article provides some insight about the different ammunition pressure standards (SAAMI, NATO-EPVAT, CIP, U.S. ARMY).

home.comcast.net/~ehorton/The%20 … 20NATO.pdf

Cheers,

Schneider.


#2

Schneider - it is an interesting study. Years ago, when I shot a whole lot of .308/7.62 x 51 NATO in many different rifles, and was not aware of any warning about non-interchangeability, I never had problem number one. Today, I have only a K98k Mauser converted in Isreal to 7.62 x 51, and shoot it seldom.

So, my esperinece would put me in agreement with him.

However, there is a troublesome sentence in his study - the first sentence of the “Conclusions” section:

 "The pressure difference between the two rounds is insignificant; the real problem is commercial ammunition has thinner cases that were not designed to shoot in military chambers BUT we do it all the time anyway and this (is) why you see more case head separations on commercial cases fired in military chambers."

He then goes on to say, just a little later:

 "It is safe to shoot .308 Winchester in your 7.62 x 51 rifles (even the Ishapores) and vice versa."

The two statements seem to be at odds with each other. He appears to be basing his conclusion on the safety of interchangeability being a matter ONLY of chamber pressure, and then presents a reason (chamber specifications) that case head separations are often seen. A case head separation is no joke in a rifle firing 50,000 CUP ammunition! It can cause damage and injury.

His statement about casehead separations is couched in terms that gives the reader the impression that it is a common occurance when commercial .308 Winchester ammunition is fired in 7.62 NATO chambers. I do not have the statistics or personal knowledge to know if this is true or not, so I am making no judgment of my own as to the safety factors involved here.
However, based on just just two statements alone in his study, my caution would be NOT to fire .308 Winchester-caliber ammunition in military-chambered rifles.

Again - not my personal call but one based simply on this particular document and the knowledge that chamber pressure is not the only factor governing the safety of interchangeability of two similar cartridges.

John Moss


#3

This subject has been kicked around more times than poor Richard Nixon. It’s time for it to go away, but I don’t think it will.

John, I noticed that same contradictory comment and sort of cringed when I read it. I’ll bet the author would take it back if he had the chance.

There’s no evidence that I’m aware of to back up the claim of case head separations. Before around 1993, the National Matches were fired almost exclusively with the M118 and M852 ammunition. When the rules were relaxed to permit the use of commercial cartridges and/or handloads, the need for government ammunition all but disppeared and it was only a few years later (ca 1996) that the M852 was discontinued. Today, shooters almost exclusively use 308W factory ammunition, or, if they handload, commercial 308W cases. There has not been an outbreak of case head seperations AFAIK.

International Palma rules have also been changed and the official cartridges for these events are the 308W and 223R.

I have but one rifle chambered for the cartridge. I use it for all of my velocity comparisons and have fired hundreds of 7.62mm ball and NM ammunition in addition to the commercial 308W and my own handloads. I chambered the barrel myself using a 7.62mm NATO reamer. I’ve not yet had a problem with it and I’ve fired some hot loads in it, including the Quantico G4 cartridges which exceed the pressure of both 7.62mm and 308W.

The Internet is a wonderful thing but there is so much bad information on it that it’s hard to seperate the good from the bad. I do not envy new shooters who read this stuff and wonder.

JMHO

Ray


#4

John
IMHO. the seperation issue is most likely to occure in something that’s belt fed (which is also where you’ll find the “loosest” chambers)


#5

I have to wonder what kind of commercial ammunition has been alleged to cause case head separation in military chambers. If someone were to shoot a lot of “Super Heavy Magnum Like” hunting loads with heavy bullets and slower powder out of their M14/M1A, there could be issues with high gas port pressures and violent/premature case extraction (as well as a trashed op rod…). Perhaps something like that led to tales of commercial ammo coming undone when fired from a mil chamber?

Dave


#6

On a related but lateral subject. Out of curiosity, what are the dimensions of the 7.62x51 CETME (Spanish)? Do they correspond with the 7.62x51 NATO (308W)? I know that the pressures for the former are lower than the latter and that the 308W will chamber in the M1916 Spanish Carbine. I’ve also witnessed first hand what it will do to the un-enlightened (a neighbor) when the chamber/barrel rebel, mangling the aforementioned’s hand. Knowing pressure points and direct pressure came in handy till EMS shows up.


#7

Ray, thank you. My experience, as I mentioned, has been the same as yours. I probably have not fired that caliber as much as you, but I have handloaded it both for semi-autos and for a 40XB-BR Remington Heavy Varmint Class benchrest rifle. Except for sizing and seating the bullet, the same basic techniques were used for both and both commercial and military match brass were used. I seldom, if ever, use straight military rifle brass, and not much pistol brass either. I prefer match or commercial, primarily because I am too lazy to cut out primer crimps.
I have been lucky all my life in finding quantities of commercial cases, usually once fired, at cheap prices, in the calibers I want to load. I never had any problems, and I had not read anywhere about any plaque of case-head separations, although in two of the “excellent” Indian conversions on the Lee Enfield, the Model 2 in India I believe it is called, we had them, both with the same person (we replaced the first rifle with a second), an expert shooter and reloader who used to contribute occasionally to this forum. We had ordered only three of the guns - we sent them all back and never would buy one for resale again.

That’s why I hedged my answer a bit.

Thanks for straight scoop that can be trusted.

By the way, regarding belt-fed automatic weapons, the only case-head separation I ever witnessed in the Browning Machine Gun (M1919A4/A6), the only one with which I have some experience, was a direct result of improper headspacing on an A4 after cleaning. It was very exciting, happening to the second gun n our LW squad. Glad it wasn’t our gun. Of course, we fired only military issue ammunition in it.

John Moss


#8

I shoot at Bisley in the company of hundreds of 7.62/.308 shooters. I can’t recall ever hearing of a case head separation, there probably have been a few but I’m not aware of any. The reason in a nutshell is because target rifles have tight chambers.

We have to examine the term case head separation in its true terms. It is not a pressure blowout which is a common misunderstanding, although pressure plays its part. It is a stress fracture in the brass. Brass is very ductile when worked slowly but becomes brittle if worked too fast and / or too much.

In any rifle case within a chamber the job of containing the pressure is done by the chamber. Right down to the last quarter of an inch or so when the job passes to the case itself. At this point the brass inside the case should have thickened to take the extra load. It is at the point of handover that a separation will occour if its going to.

At the moment of firing the case expands right down to the point where it cannot expand any more because the brass is too thick. The stress occours at that point. The molecules to the front move outwards but the molecules further back dont. The ones in between are stretched sometimes to breaking point.

7.62 cases are almost always heavier than .308 cases and 7.62 military chambers are looser and less supported than bolt action rifles.

I have seen many separations in .303 because of the incredibly loose chambers but every one I have seen is an indication of stress (cold stretching) rather than pressure per se. A rough jagged hairline crack.

Look at any fired case, preferably a 7.62 but even better a .303. Run your finger nail along about a quarter of an inch from the base and then hold it up and examine it. You will both see and feel the point at which separation occours if it is going to. A slight ridge around the case. In extreme examples if you clean the case with vinegar you may see the brass has gone matt and grainy at this point.

It may be slight almost un noticable or it may be quite pronounced but its where the brass is tested to its elastic limit. Loose chambers and sloppy headspacing will exacerbate the amount of stretching.


#9

Vince, I agree completely with your analysis. I once had an Enfield Jungle Carbine I shot a lot and reloaded for. After one or two reloads with full resizing, I would get partial (or rarely) a full case head seperation because it had a VERY loose chamber. I went to using only neck resising and and never had another problem and I got lots of reloads on those cases.

Thanks for your explaination.

Lew


#10

Here are the dimensions of the 7,62 x 51 Cetme and 7,62 x 51 Nato, from an article by Francisco Lanza in the army magazine (1960).

The case on the right shows the Nato dimensions except for the primer pocket which is that of the Cetme case, according to the author.

The Cetme case weighs 9,7 grams and the Nato one weighs 11,9 grams.

Cheers,

Schneider.


#11

Vince I agree with what you had to say also but if I may, let me pick apart as to why? this stuff happens in my opinion.
You stated:

In any rifle case within a chamber the job of containing the pressure is done by the chamber. Right down to the last quarter of an inch or so when the job passes to the case itself. At this point the brass inside the case should have thickened to take the extra load. It is at the point of handover that a separation will occur if its going to.

The job in my opinion does not pass to the case the last quarter of an inch or so, it can’t because the “brass case” is the weakest link in the whole chain of events. And even extra thick brass will not hold up to 60,000 + psi. So why does this happen?

  1. It all starts when the chamber is first cut, alignment of the chamber reamer to the rifle bore is extremely critical.
  2. There is no such thing as a perfectly straight barrel.
  3. So no matter if you dial in the headstock and tailstock of the lathe to 0.0001” does not mean that the bore “where the chamber is going to be cut” is within the same alignment as the rest of the bore. (so one ends up with a loose chamber down on the case where is shows the most)

So when one compares military rifle chambers to commercial rifle or “match grade” chambers then everything falls apart. Now everybody agrees that “military chambers” are loose or sloppy, why is this? Because of the above stated alignment issue.
You stated:

I have seen many separations in .303 because of the incredibly loose chambers but every one I have seen is an indication of stress (cold stretching) rather than pressure per se. A rough jagged hairline crack.

The pressure of the round going off is why this “rough jagged hairline crack” occurs because OF THE LOOSE chamber. Not because the brass is so different between 308 Win and 7.62 NATO. Yes that “line” is at the junction point in the case itself (the web) but if the chamber was cut correct or tighter if you will, this is some times not even seen much less felt with a finger nail.
Check it with a mic. and it will be 0.002” to 0.004” if cut correct.

Now “I” am not an expert on this but I have cut a few chambers of all calibers, and I have found that if the lathe is set up as close as one can get that machine to be, you will not have this loose chamber or in some cases “egg shaped” chamber.

I am not knowledgeable in the manufacture of military barrels but I just know in my heart that the operator just sets up the machine “as close as can be” and that’s good enough. And then when that chamber is cut another barrel is set in the lathe right away and on you go. Even the quality of the barrel steel is no where near the same quality as any commercial grade barrel.
And machine gun barrels are worst because it is known from the start that the bore will be shot out very very fast, so why worry about a “loose chamber”

I may have to stand corrected here but, I don’t know of any headspace gauge makers that “don’t have 308 Win and 7.62 NATO as interchangeable” And to put things into perspective here the difference between a “go gauge” and a “no go gauge” is about the thickness of a single layer of “masking tape” ya not much and that is the only chamber dimensions that the military barrel makers seem to care about and sometimes that does not seem to matter much.

So we shooters should have a lot more trouble than we do with this 308 Win. And 7.62 NATO interchangeable truth problem.
Just my opinion and almost not AMMO IAA related.


#12

Steve,
Good comments and I thank you for that. I have a .303 case from my own rifle I would like to post a picture but I don’t know how. If you PM me your address I would like to mail it to you. Perhaps you could have a look and tell me what you think. It has all but separated.
If nothing else it would be a good example for your collection.