Home Forums Hunting & Shooting Ask The Gunwriters Pressure 308 Win vs 7.62 Nato

I know this subject has been beat to death for a lot of years and I apologies for bringing it up again, sorry.

I have read several articles and manuals that claim the 7.62 Nato round is loaded to a Max operating pressure of 50,000 PSI, while the 308 Win is loaded to a Max operating pressure of 62,000 PSI.
I have also read just many articles that claim the Nato round is loaded to 50,000 CUP which is on par with the 62,000 PSI rating for the 308 Win.
There seems to be a lot of confusion between the CUP Nato pressure rating and the SAAMI PSI rating.
So, I am here asking if you guys can finally clear this up for me. What is the actual pressure rating of Military 7.62 x 51/7.62 NATO ammo in terms of PSI? Not CUP or some other Military, European measurement, simple PSI so it can be compared to the PSI rating of factory 308 Win.

Thanks
Steve

Strictly speaking, I think the NATO test is not the same as the SAAMI CUP test. So there is another layer of confusion.

The good news is that Monday I will be with some people who know exactly how NATO does their test. I would not be surprised if they have converted to piezo or strain by now.

You're likely to get a good clear answer from someone on the board, but if nothing pops up by Monday, I will ask.

Until about the 1960s, measurements taken with the copper crusher system were often reported as PSI because the original SAAMI position was that they were one and the same. Ackley's book is full of crusher data that he reported as PSI.

You're right it's been done to death and much of the references in the OP are more or less wrong. For a good technical discussion see http://thefiringline.com/forums/showthread.php?t=326877

One of the many issues addressed is that US Army tech manuals have continued to repeat old and obsolete figures - which are close enough for government work. This includes reporting values of 50,000 CUP as PSI long after the cartridges in use had changed and so had the technology and so it goes. Tech manuals can be more or less user's guides and not really very technical. Folks repeating tech manual information in the erroneous belief there must be some relationship to lab figures have contributed to the later confusion.

SAAMI
308 Winchester Specialized proof chamber and barrel
430 MPa / (62,000) or 52,000 c.u.p.
Proof Round
558MPa / (80,600)
Transducer located 175" behind the shoulder of the case for large diameter(.250") transducers.

NATO
EPVAT
7.62 mm(7.62x51mm NATO)
Designed to chamber NATO ammunition
415MPa / (60,190)
Proof 519.0MPa /(75,275)

Pressure recorded in NATO design EPVAT Barrel with Kistler 6215 Transducer or by equipment to C.I.P. requirements

Clear now? - as a practical matter the cartridges are measured differently and chambered differently but SAAMI has not found it necessary to make any alarming statements and for the home shooter shooting comparable weight cup and core bullets with powders that very nearly the same, maybe a cannister grade and a non cannister grade or even a handload with surplus powders and cases with and without the NATO cross usually give velocities that will be very similar. Often as close as we can come to making any statement about pressures and internal ballistics in our own rifles.

I side with the late William Davis: CUP is a goofy term that muddles the issue. "CUP" effectively means "psi as measured by a particular system known to be inaccurate." Davis preferred something like "psi(c)" to indicate the psi reading was done with copper crushers.

Any measurement system has errors. The copper crusher has for well over 100 years been known to give readings below the true pressure. It was, though, the standard through the first half of the 1900s simply because it was inexpensive and adequate. Because it was so common, that "(c)" part was inferred.

Today, a strain gauge system should prove more economical while giving more accurate readings with a far more complete picture of the pressures in the chamber. As implemented in home units, I wouldn't be surprised if the typical strain gauge installation is off by a few thousand psi, but percentage wise, that's not much and beats the crusher's 20% (or more).

This old Horse has been flogged so many times I can't believe it's still alive ! The confusion results from two different units of measurement used on the same Cartridge ! 7.62 NATO ( CUP ) .308 Win. ( PSI ) , end of story !

Steve, your little friends are wrong. They have been affected by the skepticism of a skeptical age. They do not believe except what they see.

They think that nothing can be which is not comprehensible by their little minds. All minds, Steve, whether they be men's or children's, are little. In this great universe of ours man is a mere insect, an ant, in his intellect, as compared with the boundless world about him, as measured by the intelligence capable of grasping the whole of truth and knowledge.

Not believe CUP? You might as well not believe in fairies! You might get your papa to hire men to hunt for you in all the forests around, but even if they did not believe in CUP, what would that prove?

You may tear apart the baby's rattle and see what makes the noise inside, but there is a veil covering the unseen world which not the strongest man, nor even the united strength of all the strongest men that ever lived, could tear apart. Only faith, fancy, poetry, love, romance, can push aside that curtain and view and picture the supernal beauty and glory beyond. Is it all real? Ah, Steve, in all this world there is nothing else as real and abiding.

No CUP? Dear me, there certainly is! And your question? It lives! It lives forever! A thousand years from now, Steve, nay, ten times ten thousand years from now, we will continue to talk of CUP.
---



Now, that said, forget about pressure. It's a moot point.

The only difference involves the chambers of the rifles - military and commercial. Commercial rifles have shorter chambers. Even that is being muddied by firearms makers. That can be part two of the discussion and should keep the thread going for several more daze.

http://www.303british.com/id36.html

Again, I must differ. CUP is measured in psi! Crushers are calibrated in psi using a hydraulic piston.

The problem is the crusher can't track the pressure pulse and nearly always indicates a pressure lower than peak. While not ideal, this hasn't proven to be a great problem. Advances in technology leave the system not only inaccurate but (probably) more expensive as well--it's going the way of the dinosaur.

Keep goin' guys - I have plenty of popcorn & beer!
Mark

You just want someone to blame when you're drunk! (You know this won't end before the beer runs out.)

I've also read that the ballistic numbers for the 7.62 are 2800fps/150gr bullet and 2600fps/174gr bullet. If these numbers are correct, how can a 7.62 Nato get these kinds of velocities at 50K PSI? All the 308 load data I have tells me that these velocities require much more than 50K PSI, more like 60K PSI.

Those 50 ksi numbers were measured by an inaccuate system based on the copper crusher. When measured by more modern and more accuate sensors you get 60 ksi.

It's sort of like measuring a quarter mile by eyeballing it as compared to using a laser range finder.

They were the standard at the time, and as accurate as they knew. A better method has since been found. Don't sweat the old data; ignore it.

I have wondered how consistant the CUP are. If they are consistant, it seems that it would be a simple matter to re-calibrate the gauges or copper pellets to more closely coincide with readings obtained from other pressure measuring devices.

What about LUP, as used to test low pressure firearms and shotguns. Do LUP suffer from the same inaccuracy as CUP?

I am at the Lake City Army Ammunition Plant, and, as promised did ask about pressure measurements. All pressure measurements here are done at the case mouth.

The thing that kills you in a measurement system is the magnitude of the random error in the system. That pretty much governs everything else.

For nearly 100 years, more accurate systems have been around. The crusher has held on because it was relatively inexpensive yet consistent enough to get the job done.

As Steve says, ignore the old data; but there's no need to ignore modern data shot with crushers. I have a .30-40 in the works. I'll be shooting it with Lyman's (fairly) recent data. It was shot with crushers because no SAAMI piezo standard exists. Given the choice between modern piezo data and modern crusher data for the same cartridge, I'd take the piezo tested data; but crusher pressure data is better than no pressure data at all.

As for the consistency of crushers, I can't say. Denton has noted elsewhere that crusher's have more random errors. That was consistency between shots, if I recall.

There's another "consistency," what's the true pressure between chamberings when both read as 50 ksi with crushers, say .243 vs .358. The true pressure is likely in the low 60's in both cases, but the odds are the true pressures are not equal. I doubt anyone has established any correlation.

Crushers have proven consistent enough to allow companies to make safe guns since the late 1800s. While not an ideal measurement system it proved adequate.

Here's one interesting tidbit: guns proofed with crusher standards may be more robust than those proofed with piezo. Some 1960's data taken by a fellow named Brownell indicates that crushers fall further behind the true pressure the higher the pressures go. Since crushers and piezo both proof to the same percentage overload, it may be that crusher proofs are more severe than piezo proofs.

well....drats. I had hoped to find a load that worked well in both my AR10, and in a bolt .308.

I loaded & shot my first handloads in the AR10 this past weekend. They shot accurately, but did not function well. Some rounds failed to eject, and the primers fell out of a couple. The brass is Federal .308 Match, which had been once fired in the same rifle. The powder charge was Hodgdon's starting load of Varget. I am starting to wonder if I have a soft brass issue, or a headspace issue, or both.

So does anyone sell match brass in 7.62, sized for the military chamber?

You should be able to measure your fired brass with a headspace gage http://www.midwayusa.com/viewProduct/default.aspx?productNumber=479704 and adjust your 308 FL die for proper shoulder bump.

For another go round on this dead horse with comments by nationally recognized experts with broad experience and easy to find material that forms a useful background to beating this long dead and rendered horse see: http://www.fulton-armory.com/faqs/M14-FAQs/308.htm





Military issue match loads are readily available; even I have some as gifts from military shooters and in the past - and maybe today for all I know - the once fired military brass has been fairly easy to get by swapping commercial used with once fired by the military teams - the military shooter has to turn in used brass but in the past there was no requirement that it be the same brass the military shooter fired.

That's one reason Forster and PTG and others make and Brownell's and Midway and others sell a set of multiple headspace gages covering a range of chamber lengths by thousandths.

Federal is notorious for in some years not catering to the reloader and so reuse of their cases can be disappointing - in other years Federal brass has been great so users will report vastly different experiences with good cause.

I'd suggest good new NATO marked brass - Federal has been selling brass out of Lake City at Midway and all the other vendors (Graf's, Widners and all the usual suspects). In any event loads perfectly adequate for all but 600-1000 yard use by high masters can be found that for most people work across gas, bolt and even double rifles (mine is a relative cheap Valmet). Maybe if I were a high master I'd find a load that worked for long range too but I can't say.

Finally I'd ask if anybody reporting problems with a military style rifle has actually gaged to know what chamber is in the given rifle. FREX in 5.56/.223 with the obvious exception of Colt made mil-spec AR type rifles any number of people have found their own rifles chambered in ways they didn't expect - either way -

Finally to repeat myself to the best of my knowledge and belief the figure of 50,000 be it CUP or given as PSI was simply a carry over and had no particular relationship to any then current actual measurements.

Reloading for gas guns - for match purposes - is well covered all over the place most especially in the Precision Shooting guide, see also the OP Sinclair guide and so many others. It's easy to find loads in many a manual that are specifically for gas guns and yet many of the same loads are also listed and used in other firearms.

Funny, the Fulton Armoury article looks like what I wrote years ago. Naturally, it would be. Professionals know stuff like that.

But wait...

I'm not an pro, merely an amateur in the field I guess. I'm glad you pointed that out. I'm only a retired military armourer with books in use by the firearms forensics branch of the govt here as well as police departments around North America.

I thank you for posting the real truth.