.308 Win. vs 7.62 NATO

J

Jim Watson

New member
It has become common currency in the cruffler community to warn against shooting "high-pressure" .308 Win. ammo in 7.62 NATO rifles. After all, it's right there in black and white: Chamber presure; .308 Win., 62,000 psi; 7.62 NATO, 50,000 p.s.i.

But wait, my NRA Handloading guide (1981) says: "The maximum average chamber pressure for the 7.62mm NATO military loads must not exceed 50,000 p.s.i., measured in the military pressure barrel for COPPER-CRUSHER pressures. The commercial .308 Winchester cartridge is normally limited to a working pressure of about 52,000 c.u.p., and the maximum product average for commercial ammuntion should not exceed 55,200 c.u.p."

This says to me that the mil-spec still uses the old terminology of reporting crusher readings in p.s.i. and has not adopted the SAAMI term "CUP." And that the pressures are not greatly different. From pressure readings I have seen, there will often be more than 2000 psi/cup round-to-round difference in a single string and more than 5200 from lot to lot or barrel to barrel.

I think the two rounds are equivalent and we are just caught up in changes in test procedures and terminology.

Don't agree?

OK, if the .308 has an allowable pressure of 62,000 psi and if the 7.62 only 50,000; and assuming they were measured by the same system (which I doubt), that is a 24% increase in .308 over 7.62. Why, then, does the .308 not give substantially higher velocity than 7.62? A 24% increase in pressure would correspond to a 10-12% increase in velocity, according to the Hutton-Powley or VihtaVouri rule of thumb. I don't think the ammo companies would miss the chance.

Explanations, references and test data will be appreciated.

Note, I am not saying that the FR-7 is not a 7.62 CETME of the same case shape but lower pressure rating. I also know that the larger chamber dimensions of 7.62 and thick military brass will affect handloading. It just looks to me like the .308 is the full commercial equivalent of the 7.62, no less, no more. Just like the .30-06 before it. At least it didn't have to get by under two names.
 
To address a very small portion of your post.

Higher PEAK pressure may or may not render higher velocities in the same test gun with the same projectile.

Burning charactoristics of different powders make the peak pressure figure more of a safety determinant than a performance indicator.

A classic example I like to use in illustration comes from Winchester loading data wherein with all other factors being equal, a power change rendered 400 ft sec slower projectile while increasing PEAK pressure by 10,000 pounds.

Sam
 
Most of of what I read says not to use military ammo in a sporter rifle because it could blow up the rifle. I don't know if its true but I have read it more than once. Does anyone know if thats true? The price on the nato spec surplus sure looks good for plinking.
 
I've used tons of military and factory .308 in my M1A and there's been no problems. So has my buddy in his. Also works fine in bolt guns(not as accurate as match stuff but no pressure problems either). Just shoot it and don't worry about it.
 
You'll be fine with most commercial .308 in a 7.62 NATO gun, just be careful of stuff like Hornady Light Magnums... Things like that usually are marked "for bolt-action rifles only" or otherwise indicate that they are not suitable for gas guns.
 
I once tried some military surplus 7.62 in my Savage .308, and had one heck of a time extracting the empty casings. I would have to pull and bang on the bolt to get the empty case out. I ended up giving the ammo away.
 
I'll admit to having propagated this info on occasion ... I thought that it was accurate.

CR Sam's point about peak pressure has no argument ... I've always searched out the powders that for my particular application give the best results with the lowest pressure ... slow burning 2400 in a 6" .357 is a great example of lower peak pressures (but probably higher average pressures) giving superior performance to the faster burning powders that are probably better for the snubbies.

But ... I, like Jim, have a hard time believing that Winchester, Remington, Hornady or any of their peers will let their engineers go home if they settle for a powder selection that will impulse peak at the max pressure chamber and then drop to 25% of that pressure to send a round lollygagging down range. While some may assume that the light magnum rounds are higher pressure ... my guess is that this is true only if you are talking about average pressure and not peak pressures. I don't have the data ... but I'd love to see it!

Are the NATO rounds different from the commercial .308? I don't know ... fortunately, I own all .308 rifles so my interest is more academic than anything ;)

Saands
 
Brent:
On occasion I have noticed that case sticking phenomena. I think that I have correlated it pretty closely to the corrosion that exists on some of the not so pretty surlpus out there. Like I said, this is a correlation ... and not necessarily a causal relationship! It could be that the better cared for surplus was also manufactured more carefully ... but since I have started being more picky of my surplus sources, I haven't observed the problem.

Saands
 
I don't have links or references at hand, except to mention that this subject has been hashed over pretty thoroughly on the FAL Files in the recent past. (The FAL Files is back up, BTW; it was down for a few days.)

My understanding is, the rounds are effectively equivalent. The 50,000 psi specification for 7.62 NATO is measured by copper crusher. The 62,000 psi SAAMI spec for .308 is measured electronically. Electronics are quicker to respond to pressure and will read the peak better, giving a higher reading. Copper is slower to react and will miss the actual peak. Thus the difference.

As I recall, the case thickness specs are different, the military round has thicker brass. Also the chamber specs are slightly different. The mil spec chamber is a little bigger on both the high and low end of the spec tolerance. A big military chamber can be within mil spec, but above commercial spec. A tight .308 chamber could be below mil spec, slightly. (I'm working from fallible memory here, so a double-check of this would be wise.)

As everyone probably knows, the case expands when a round is fired. That's good, since it seals the chamber. After the pressure drops, the brass relaxes a little and comes loose from the chamber walls. That's necessary to extract the round.

For the case sticking problcs are slightly differenem, then, I wonder if it's a combination of a tight .308 chamber and the thick mil brass. When the case expands, it doesn't have room for that thick brass to spring back and come loose from the chamber wall. Maybe?

Hornady says their Light Mag load doesn't develop any higher than normal pressure. I believe the way the LM works is, they use a larger amount of slow powder, and keep the pressure up better as the bullet goes down the barrel. Hornady mentions on their web site, it takes a long tube to get good results with the LM. A 24" or 26" barrel would be the way to go.

So I think the LM problem with semiautos, is not that the chamber pressure is high, but the pressure will be too high down the barrel at the gas port.
 
Thanks, JPaul. I don't do FALs and was not aware of that site. It looks to me like Uncle Buck and the boys have it straight. I was just getting pounded by the urban legends from other sources and it was driving me crazy.

The only thing I saw wrong was the confusion between piezo transducers and strain gauges. As I understand it, the piezo transducer generates a signal by the voltage generated by mechanical distortion of a quartz crystal in a probe screwed into a port in the barrel. A strain gauge is a grid of wires cemented to the barrel. Its signal is a change in resistance as the wire is stretched by the barrel actually expanding under pressure. That is why you can get a do-it-yourself strain gauge kit and put it on your own gun with no gun modification except for having to sand off a patch of blue to get a good bond.
 
I own a NEF in 308 and I can tell you first hand that there is a difference in the surplus and the commercial as far as extraction. I can only use the surplus as long as I keep a cleaning rod handy to knock out the spent casings. They will not eject. The commercial will extract smoothly each and every time. I too think it is a difference in the thickness of the brass.
 
DML,

A page at the link you posted says--

"Some Final Thoughts
Military ammunition is loaded to maximum average pressure 50,000 PSI using the modern piezo transducer method of measurement."

That disagrees with a statement on a document found by Uncle Buck (his FAL Files name.) The doc is:

MIL-C-46931F(AR)
29 March 1991

It says:
=======
"3.7.1.1 Chamber pressure measurement at 70°F by copper-crush
cylinder test method. The average chamber pressure of the
sample cartridges conditioned at 70°F shall not exceed 50,000
pounds per square inch (psi). ...
.
3.7.1.2 Chamber pressure measurement at 70°F by EPVAT test
method. The average chamber pressure of the sample cartridges
conditioned at 70°F shall not exceed 365 MeqaPascals (MPa) (52,940
psi) . ..."
=======

Unless the military has changed specs since '91, which I doubt, the 50,000 psi spec is not by piezo as the web page says, but rather by copper. (EPVAT is the electronic method.)

I've seen the same statement on another website, that the 50k measurement is piezo, but I believe that's wrong.

I believe the NRA Handloading Guide is correct about the pressures, as Jim Watson posted in the first message above. The NRA Guide and this military doc agree about the 50k being a copper method measurement.

No argument that the chamber and brass specs areaPascals (MPa) (52,940
different. Also, no argument that the commercial .308 pressure spec is a little higher.

But if there's a problem with using .308 in a 7.62 rifle, or vice versa, it's going to be because of brass, or chamber size, and not a "raw" pressure difference.

So that's my story, and I'm sticking to it. :)
 
When the .308 first came into the civilian world, Remington stated in their data sheets of cartridge performance that it was loaded to a chamber pressure of 53,000 psi.

Rifle designs are commonly proof-tested to 150% of rated safe pressure levels. This pressure level might ruin a semi-auto's operating system, but it won't "blow up" the action/receiver of a gun. Note that the push-feed action with its fully-supported case-head will allow proof loads exceeding 150%--which is why they were built in the first place.

Whether factory or "at-home" loadings, the idea is that the life of a rifle is generally determined by the flame erosion of a barrel, not by over-stressing of the bolt such that headspace becomes a problem.

FWIW, Art
 
Jpaul:

I simply posted the web page for what it's worth. I am no expert and have nothing to do with the information provided.

My understanding of one difference between the 7.62X51 and .308 Win. has to do with the pressure at the gas ports. I think there are good reasons not to load slow burning powders for ammo intended for gas operated rifles. I'm sure there is more to it then that, but as I said, I am no "expert".
 
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