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Old January 24, 2001, 10:09   #1
TideWater 41009
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Specifications: 7.62mm versus .308 Winchester (diagrams)

According to the March 2001 issue of Guns & Ammo magazine, and quoting from page 26, regarding the .308 Winchester; "Chamber pressures are 52,000 cup by the copper crusher method and 62,000 psi using the transducer method. That's up slightly from the values for the .30-06, but a little below maximum magnum pressures".

There seems to have been a lot of interest in this subject in past postings. I think there has been some confusion resulting from getting cup and psi mixed up. Hope this helps.


[ November 20, 2001: Message edited by: EMDII ]

Last edited by TideWater 41009; December 01, 2003 at 23:50.
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Old January 24, 2001, 14:24   #2
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Yes, this topic is of interest to me. Appreciate you posting the info.

When I get around to it, I'll holler at a few ammo manufacturers to see if they load their 308 to 62,000 PSI MAP or a little milder in deference to arms chambered for 7.62 NATO.

Again, thanks.

Scott
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Old January 25, 2001, 15:21   #3
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Got replies from Hornady and Federal. See my reply to JoeDoe's "New to FAL's" post in the General forum for the scoop.
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Old January 25, 2001, 15:55   #4
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Very good discussion of the difference in the cartridges. Is your point then that reloaders should try to duplicate the military load? Any suggestions?
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Old January 25, 2001, 17:45   #5
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The point is that you get a different pressure number for .308 Winchester depending on which gauge you choose to measure it with. You get 52,000 PSI with the copper crusher gauge, which is known as copper units of pressure (cup) and you get 62,000 PSI for the same loading of the same .308 Win with the piezoelectric transducer gauge. The use of these two different gauges by the US military (copper crusher) and the commercial SAAMI specs (piezo) is responsible for the fictitious net rumor that .308 is loaded to much higher pressures (62,000) than 7.62 NATO (50,000). They are actually loaded to approximately the same pressures when you measure with the same gauge (50,000 vs 52,000 PSI). Die rumor, die!
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Old January 26, 2001, 13:10   #6
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Uncle Buck, I think your post isn't entirely accurate. Let me try to explain.

You are correct that you get different numbers depending on the measuring method you use. However, you can't get PSI from the copper crusher measuring method, nor can you get CUP from the piezoelectric transducer method. When one uses the copper crusher method, one *always* gets a pressure value in copper units of pressure (CUP). When one uses the piezoelectric transducer, one *always* gets a pressure value in PSI. There are no exceptions, nor is there a formula to convert from CUP to PSI, or vice versa.

So, the first post in this thread, from RefinishGuy, is accurate. It states that the chamber pressure for .308 Winchester is 52,000 CUP or 62,000 PSI, depending on the measuring method. Those two values are approximately the same; it's just the units of measure are different. It's kinda analogous to measuring the length of an object with a metric ruler and a standard ruler. A .308 cartridge case is 51mm long using the metric scale and 2.010 inches long on the standard scale. Same length, different measuring units.

So, if we compare apples to apples, it seems we do find that there is a difference between the .308 and the 7.62. Per the first post, .308 Winchester is loaded to 62,000 PSI (as measured by piezoelectric transducer). Per page 11-3 from the Army Tech Manual TM43-0001-27 (June 1981) that Dano posted on September 10th of last year, the 7.62 is loaded to 50,000 PSI (also measured by piezoelectric transducer). Comparing pressure of the two rounds as measured by copper crusher, the .308 is 52,000 CUP and I the 7.62 is around 44,000 CUP (sorry, can't cite a source for 7.62 in CUP, but I remember seeing it somewhere around that figure).

For confirmation of this info, check out page 57 of the online reloader's guide at www.AlliantPowder.com. That page lists the pressures, as measured in CUP *and* PSI, for several common rifle loads shown in their guide. They list their loads for the .308 at 52,000 CUP and 60,000 PSI (2000 less than SAAMI spec on the PSI scale). A couple other loads they list are:

30-06 at 50,000 CUP and 60,000 PSI
7mm-08 at 52,000 CUP and 57,500 PSI
.222 Rem at 46,000 CUP and 50,000 PSI
7x57 Mauser at 46,000 CUP and 51,000 PSI

Although the CUP and PSI numbers are different for each cartridge, the pressure is approximately the same. (I say approximately because there are slight accuracy variations that make exact measurement essentially impossible.) I listed those loads to show that there really is no way to convert CUP to PSI or vice versa. Though the .308 and the 7mm-08 read the same pressure in CUP, 52,000, each cartridges' value in PSI is different. I dunno why.

Did all that BS make sense to anyone?

FINNWOLF, to answer your question, I think it's probably a good idea for reloaders to stick to milder loads for the FAL to extend brass life and perhaps the life of the gun. While I do reload and brass is a concern for me, my bigger concern is the gun itself. My point there is that I haven't found enough info to state conclusively that shooting .308 in an FAL really isn't a good idea. Is it worth all the ink I've spilled to worry about it? Maybe not. Perhaps I should just shut the hell up and shoot.

Thanks for the input/questions.

Scott
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Old January 26, 2001, 18:26   #7
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Your understanding is pretty close, but there still are some inaccuracies. First of all, all pressures are in units of force per area and in the US that is pounds per square inch. It is Newtons per square meter in the metric world. The differences in the gauges measurements arises from the different way they perform in a kinetic measurement (rapidly changing pressure) versus their static (constant pressure) performance, which is how they are calibrated. The Piezo gauge more accurately reflects its calibration in a kinetic measurement.
Since the copper and piezo gauge respond differently, they read different pressure values. Both values are in PSI, but to make it clear that the measurement methods have different values, the copper method is called copper units by convention (and the units of pressure are always PSI). The piezo method is not always specified, since it is more accurate, but it should be called piezo units or transducer units. There is no simple mathematical transformation for the two measurements, so a direct comparison of the numbers is invalid, although you used to find this comparison used on the net to claim that .308 is loaded to much higher pressure than 7.62. This is a net rumor that needs to die.
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Old January 26, 2001, 20:27   #8
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UB, I'm trying to understand what you're saying. (I'm an English major, so please bear with me.)

I don't consider Guns & Ammo the ultimate authority on guns, but I assume they pulled their 62,000 PSI number for .308 chamber pressure from SAAMI specs or a source that has access to the $35 book with SAAMI specs for common US rifle cartridges. Surely they aren't trying to mislead their readers. Sound reasonable?

The US Army source I cited in my earlier post lists the pressure for the 7.62 as 50,000 PSI. That spec says nothing about CUP, but on the other hand, nor does it state what measurement method was used to come up with the 50,000 PSI number.

So, given that both these sources *seem* reliable and the difference between the two numbers is 12,000, are you saying that the Army is using the copper crusher method but reporting their findings in PSI and that's what accounts for the difference between the cartridges? Or have I missed your point entirely?

If the Army is using copper crusher and reporting that as PSI, that sounds dangerous. It's also contrary to all labeling of US cartridge pressure data I've seen. When a manufacturer measures using the piezoelectric method, PSI is the only label used.

I do understand that there is inherent inaccuracy of "measuring" dynamic pressure by comparing to static calibration. However, the 2,000 PSI difference you cite is fairly significant, much less 12,000 PSI. I don't think inherent error can account for that much difference.

I look forward to reading your reply.
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Old January 26, 2001, 22:04   #9
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I trained in physics :-) Remember, pressure units are always force per unit area. This is just one of those laws of physics, just like units of speed are always distance per unit time. Listing a number as PSI doesn't tell you anything about how the number was measured. The US military continues to use the copper crusher gauge, since they have used it for 100 years, so their PSI numbers are all cup. SAAMI uses the more accurate piezo transducer gauge, so their PSI numbers are different and not directly comparable to cup PSI numbers. The point is that if you don't use the same gauge or measurement system, the comparison of the numbers is not valid, since one gauge reads 52,000 PSI and the other reads 62,000 PSI on the same firing.
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Old January 27, 2001, 12:04   #10
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OK, now I see what you're saying. So, although the US Army lists their measurement as 50,000 PSI for the .308, they use the copper crusher method to take their measurement, and they mistakenly label that measurement PSI instead of CUP.

Although that sounds seriously suspect to me, I can't refute that. I'll look for info that states exactly what measurement system the US Army/military uses and report if I find anything.

I do take issue with your statement, "Listing a number as PSI doesn't tell you anything about how the number was measured." Perhaps this doesn't hold true for the US military or for European countries, but here in the US, per SAAMI, labeling cartridge pressure as PSI means it was measured using the piezoelectric method, CUP by the copper crusher method, and LUP by the older lead crusher method. Reloading manuals do not deviate from this practice. I can't imagine the consequences of measuring a .308 rifle load at 51,000 units of pressure using the copper crusher method, labeling that value as PSI, and publishing that finding. Novice reloaders might then assume that the top load listed reached only 51,000 PSI, 11,000 PSI short of SAAMI max, and try to load hotter, with perhaps disasterous results.

I do agree with your statement that one can't compare cartridge pressure values measured with different methods.

Good discussion. Thanks.

Scott
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Old January 27, 2001, 13:03   #11
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Unless Uncle Buck is saying that the Army tested using the copper crusher but listed the pressure using the PSI equivalent of the CUP rating. Either way, I have to believe that if the difference were that great, all the books and magazines and ammo retailers would not use the terms so interchangeably. Right? If it was actually 12,000 units of difference, I think they'd go to much greater pains to distinguish the two.

A good example is the 8mm Mauser discussed in an old thread. The weak 1888 (.318) loading is simulated in the US by keeping it at 37,000 CUP. Full power .323 loading data pushes it to 50,000 CUP. So if a difference of 13,000 CUP is considered a very dangerous difference between those two loadings, you would think the same would apply to 7.62 Nato and .308 Win.

PS I got the above info from Speer #11 manual.
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Old January 27, 2001, 13:19   #12
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The conceptual problem that many of you are still having is that all these gauges give pressure numbers in PSI. That is because they are all calibrated in PSI and we always use PSI in the US as units of pressure. The problem is that just because you see a pressure listed (and it is always PSI in the US) it doesn't always tell you how it was obtained.

50,000 cup is 50,000 PSI obtained with the copper crusher gauge.
50,000 lup is 50,000 PSI read with the lead crusher gauge.
50,000 PSI with the piezo transducer is ambiguous because they don't tell you how it was obtained. The reason that they don't list the method is that the dynamic readings obtained with this gauge are very close to the actual static calibration in PSI. Still, to avoid invalid comparison problems, the method should have been listed, like piezo units of pressure or transducer units of pressure.

If you want an analogy, it is like having two different tire gauges, a pen type and a dial type. The two gauges may give different readings on the same tire because they have different inherent accuracies, but they both read in PSI.
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Old January 29, 2001, 10:32   #13
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If all these gauges give pressure numbers in PSI, then it seems we have a little 12,000 PSI discrepancy between the figures published by Guns & Ammo:

According to the March 2001 issue of Guns & Ammo magazine, and quoting from page 26, regarding the .308 Winchester; "Chamber pressures are 52,000 cup by the copper crusher method and 62,000 psi using the transducer method..."

According to your last post, the 52,000 figure G&A lists is actually PSI.

If all these readings from manufacturers and the military are in pounds per square inch (PSI), then why do they persist in using the terms copper units of pressure (CUP) and lead units of pressure (LUP)?
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Old January 29, 2001, 16:30   #14
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That is correct: the copper gauge reads 12,000 PSI lower than the piezo transducer gauge for the same firingof a single cartridge when they are both used simultaneously. The military probably uses the old crusher gauge because it makes it easy to compare the performance on newly manufactured ammo to the standard when 7.62 NATO was originally adopted and the only way of measuring pressure was with the copper gauge. SAAMI probably uses the transducer because its reading is closer to what the peak pressure actually is.
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Old January 29, 2001, 21:57   #15
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OK. So can we get somebody to take a few rounds of Radway Green 7.62 and run it through the copper crusher and the piezoelectric thingy? Then we run a few rounds of Winchester 150 grain silvertips throught the same instruments. Then we compare the data.

All this technical and theoretical stuff sounds fine. Can we get some empirical DATA??? Seems to me that should settle the issue.
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Old January 29, 2001, 23:46   #16
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Scott S was orginally right for the most part. The copper crusher method uses a small copper cylinder that is exposed to the chamber pressures. When the cartridge is fired, pressure from the cartridge crushes the copper cylinder to some degree. Basically, the cup measurement is based on the change in length of copper cylinder after it is crushed. It is not in units of pressure but in units of deformation I believe. The problem with the cup method is that it only gives you a correlation of peak pressures to that of other cartridges not actual pressures.

The transducer method uses a strain gage that is attached to the chamber, and actually measures the strain in the barrel around the chamber as the cartridge is fired. Through a bunch of equations, it can then be determined what the pressure must be in the cartridge. The nice thing about this test is that is gives you the actually pressure curve over time and not just a correlation of peak pressures. The cup test is easier to do and has been around longer than strain gages. As far as I know there is no equation to convert from one system to the other since the measurements are based in completely different units(pressure versus length). Once again to clarify, the copper crusher method does not measure actual pressure, but gives a correlation of pressures among different cartidges.

As a side note, PSI stands for pounds per square inch. The cup number is not in units of psi but in units of cup.
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Old January 30, 2001, 02:04   #17
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Jason,
While you are correct that the copper gauge directly measures the length of a copper slug, the piezo gauge only directly measures a voltage across a strain gauge. In order to convert the direct length or voltage measurement into a pressure, you have to calibrate the gauges. You put a known (static or constant) pressure on the gauge and record its length or voltage. You create a table or list of pressure versus length or voltage to convert the direct length or voltage measurements into pressure (remember; if it is a pressure, its units are must be PSI no matter what gauge is used). Copper units of pressure or cup are PSI just like the numbers you get from the transducer gauge are PSI.

Gary,
The empirical test has been done and it was published in the 1985 volume of the Lyman handloading manual. They did simultaneous transducer and copper gauge measurements on .30-06 and showed that the copper gauge reads 12,000 PSI lower than the transducer gauge. This is rocket science.
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Old January 30, 2001, 10:45   #18
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UB,

Does it strike you as odd that Lyman came up with 12,000 PSI difference using the two pressure testing methods? If they are measuring PSI with both methods, why not calibrate the instruments to read closer together? Given the chamber pressure for the 30-06 is around 60,000 PSI (as measured by piezoelectric transducer), 12,000 PSI is a whopping 20% error. Surely Lyman (and other manufacturers) can calibrate their instruments better than that.

I still say either I'm drunk or you're wrong.

Gary,
I've been looking for some empirical data on the pressure testing method employed by the US military, but I haven't found any, yet. When (if?) I do, that should settle this issue. The closest thing I've "found" is the page from the Army manual that Dano posted last September, stating that the 7.62 is loaded to 50,000 PSI. However, it didn't state what measurement method was used (though I think it was the piezoelectric transducer method, not the copper crusher method).

As for testing at the range, both those methods are beyond my fiscal means. The only thing I can think of is to shoot both 7.62 and .308 loads on the same gas setting and see if the brass is thrown different distances. While that wouldn't tell you what the pressure is, it might give some indication of whether one load is hotter than the other. Or it might not.

I'm open to ideas.
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Old January 30, 2001, 11:14   #19
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Scott,
The discrepancy between the two gauges has to do with their different response factors when measuring a kinetic or rapidly changing pressure. If the gauges were only used to measure static or constant pressures, like the pressures that were used to calibrate them, they would give the same pressure value. In a kinetic measurement, the idea is to measure the peak pressure. The response time of the copper gauge is much slower than the piezo gauge, it it misses the peak. The piezo gauge is much faster, so it reads the peak more accurately. The situation is exactly analogous to using an analog DC voltmeter and an oscilloscope to measure a 60 cycle AC voltage. The voltage changes faster than the meter needle can swing, but the electron beam of the oscilloscope has no problem keeping up with the voltage change.
So, to summarize the different readings are not due to calibration or inherent accuracy differences, but due to the different response times of the gauges when they are used to measure a rapidly changing pressure.
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Old January 30, 2001, 11:25   #20
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When all is said and done, regardless of how the pressures are measured, the average commercial .308 Winchester loading with a 150 grain bullet is listed at 2830 fps out of a 24" barrel, which is almost exactly the same as the 147 grain NATO load (although the barrel length is not given for the NATO load IIRC). Therefore, I think it is reasonable to surmise that the pressures must be nearly the same too.

Even so, pressure curves could still vary somewhat from one manufacturer's ammo to the next, and might require different gas regulator settings or eject the fired brass to a lesser or greater distance.

Or so it seems to me.
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Old January 30, 2001, 11:41   #21
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And then there's the small matter of the slightly greater case capacity of commercial brass versus the thickwalled military brass. Anyone remember the warning in reloading books about using military brass? You're supposed to reduce charges because the slightly smaller combustion chamber could yield increased pressure.

Now you can keep on arguing about CUPs, PSIs, LUPs, and BVDs, but I'm going to continue to use 7.62N and .308 Win interchangeably.
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Old January 30, 2001, 13:49   #22
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The nearly equivalent ballistics of 7.62 NATO and commercial .308 Winchester are the reality check on the fact that the pressures are the same too, even though the published pressure numbers are way different. The commercial ammo manufactures would have to be real idiots to load their ammo to 12,000 PSI higher pressure to achieve absolutely nothing in terms of performance. Attorneys would be rejoicing at their stupidity. You can be sure the manufacturers are not stupid or the lawsuits would have bankrupted them by now.
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Old January 30, 2001, 17:35   #23
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Quote:
Originally posted by gary.jeter:
OK. So can we get somebody to take a few rounds of Radway Green 7.62 and run it through the copper crusher and the piezoelectric thingy? Then we run a few rounds of Winchester 150 grain silvertips throught the same instruments. Then we compare the data.

All this technical and theoretical stuff sounds fine. Can we get some empirical DATA??? Seems to me that should settle the issue.
What are you trying to do?
Confuse us with facts?
What, and ruin a good argument?
You Bastard!

(actooooly, it's a good idea.)

My understanding, (dim), is that one of the
reasons SAAMI used the transducer guages is
that it's much quicker and cheaper to do the tests.

Good thread, keep it up.

WireGuy


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Old January 30, 2001, 18:38   #24
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Is there a formula to convert BAR to PSI or vice versa?

If so, I have some interesting findings. If not, I'm hitting the bottle again.
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Old February 01, 2001, 16:52   #25
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I found a few places with BAR to PSI conversion formulae, so it seems one can convert between the two. I used the conversion computer at www.cruffler.com (the conversion factor is about 14.5 PSI per BAR).

Tool over to http://www.border-barrels.com/pressure/pressure.htm

There you'll find info on pressure testing of throat variances that "Pressure Trials Consortium" performed on the .308 Winchester cartridge. As part of their control group, they measured the pressure of Radway Green. You have to download the zip file to get at the info, but there's one chart, Pr-rg.gif, that shows pressures of RG tests in BAR. The peak mean pressure shown for RG in the control barrel is about 3650 BAR (I say about because the graph lists pressure increments in 200 BAR, and it's kinda difficult to tell exactly), which equates to 52938.87 PSI, per the Cruffler computer.

They do list the test instrument as a piezoelectric transducer. However, their test methods conform to the European CIP standard. I don't know if the CIP piezoelectric transducer measurements are comparable to the SAAMI piezoelectric transducer measurements. If they are comparable, we have a pressure difference of around 9,000 PSI between RG 7.62 and SAAMI MAP for .308.

Interestingly, the webpage http://www.yugoimport.co.yu/products/caliber_762.html lists a type 1 proof load for the 7.62 NATO as 60,900 PSI. Unfortunately, that page doesn't state what test method they use.

Comments?
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Old February 01, 2001, 17:59   #26
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Scott S,

I remember reading somewhere, probably an old Guns'n Ammo, that there is a difference between the European CIP and SAAMI transducer methods. Are they trying to make things difficult or what?
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Old February 01, 2001, 19:53   #27
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Wadman,

You're right, there is a difference in the way the two methods measure the pressure. Question is, are those differences so significant that comparing PSI measured by each system is worthless? My guess is that UB will say yes. My answer is that I simply don't know. They are making it hard on us.

FYI, I read several posts from the thread here: http://ns.shelfspace.com/~c-r-ffl/ar.../msg00409.html

Seems this group also looked at this question and found similar barriers to uncovering comparable pressure information. One guy even wrote SAAMI and a slew of US ammo manufacturers looking for answers; poor fellow came up with essentially nothing for all his work.
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Old February 06, 2001, 12:52   #28
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Hey UB, I agree with some of the things you have discussed, but I am still certain there is a real difference in pressures between sammi and nato loads. Yes strain gages can be calibrated by using known pressure loadings, and this does make your measurements more accurate. However, most strain gages come with listed gage factors which allow you to calculate strain with a measured difference in voltage. This however is somewhat trivial to the original discussion.

I am still convinced that the copper crusher method gives units of cup which are not a pressure unit but a yardstick unit of sorts. Also, pressure does not have to be in units of psi. There are a large number of different units for pressure, such as BAR, Pascal, psi, atm, torricelli, or inches of mercury. Naturually all these different units of measure are all not equal to each other. For instance 1 atm = 14.7 psi. So even if cup was a unit of pressure, there is nothing to prove that 1 cup = 1 psi.

I found this article here http://www.reloadbench.com/gloss/cuppsi.html
which seems to clear up this issue pretty well with regards to differences between cup and psi as measured using a strain gage.
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Old February 07, 2001, 10:34   #29
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Uncle Buck is correct when he said the old CUP method of measuring pressure was in units of PSI. When the CUP method was developed, the ballisticians believed that the CUP pressure tests were accurately providing PSI data. It was only relatively recently learned that their test results did not provide actual chamber pressure in PSI.

The copper crusher method is still used and is a valid and usefull means of ammunition development, but it does not actually express pressures in PSI. When ballisticians became aware of this they began to use the more descriptive term CUP instead of PSI, but the term PSI had become so widely used that it was commonly interchanged with CUP OR LUP. This was technically wrong but since everybody was talking about the same thing (CUP), it didn't matter.

When the electric strain gauge became available the actual chamber pressure in PSI became known and is now commonplace. Still, many data labs still use the crusher test system, including the US military. EACH SYSTEM PRODUCES DIFFERENT NUMBERS FOR THE SAME PRESSURES. Confusion has resulted from these souces that do not bother to provide the correct CUP or LUP designation.

When using similar weight bullets, the .308 Winchester and the 7.62 NATO are identical, for all practical purposes, in ballistics and pressure. If you still do not believe this, ask yourself, "Why would commercial manufacturers load to higher pressures to achieve the same ballistics?"

The European manufacturers' tests are done differently, and only further add to the PSI confusuion.

Interesting thread. Thanks to all responses.

Refinishing Guy
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Old February 07, 2001, 10:56   #30
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And to add to Gunthing's post....

If the difference was truly 12,000 units of pressure (CUP,PSI,LUP, whatever), wouldn't the ammo manufacturers take greater pains to distinguish the two? After all, they DO take pains to distinguish .38SPL and .38 +P SPL which has less of a difference than 12,000 units of pressure.

================================================== =======

I dont' know if this issue will ever be adequately resolved. In the end, it's going to come down to what you feel comfortable with. My rifles are marked .308 Win and DSA even commented on this board that their rifles are chambered using those specs. Either way, these are versions of military weapons with a reputation for toughness and longevity. I'm certainly comfortable using either type of ammunition. If you aren't, then stick with 7.62 Nato and don't worry about anything else. You're certainly not going to hurt anything being conservative.

[This message has been edited by Wadman (edited February 07, 2001).]
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Old February 07, 2001, 11:03   #31
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And therein lies another problem. What are the ballistics of NATO and 308 loads? Unfortunately, I have no chronograph, otherwise I'd happily test several different loads for velocity.

I don't think that 7.62 loads achieve the same velocity as US manufacturers' .308 loads. Close, yes, but not the same. I read an article in Shooting Times (summer of 2000? don't remember the month, tossed the issue months ago) where the author, Rick Jamison, did a comparison of velocity and pressure in .308 Winchester loads. Interesting thing was that pressures well over SAAMI limits didn't make much difference in velocity. So, stating that 150 grn 308 loads at 2780 fps and 147 grn 7.62 loads at 2750 fps develop similar pressures can be inaccurate. The two loads might be very near the same pressure, but on the other hand...
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Old February 07, 2001, 11:26   #32
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Wadman,

Seems like your supposition makes sense, but remember that commercial, US ammo manufacturers are loading to SAAMI specs for firearms manufactured by SAAMI members and military ammo manufacturers are loading to mil specs for mil spec guns. The military doesn't fire commercial ammo in its guns, so there's no need to include warnings on military firearms about firing commercial ammo that might develop higher chamber pressure. Should Entreprise and DSA, because their guns are of military origin, include warnings on their guns about firing 308?

Hmmm. Who loads 7.62x51 for the US military? Do any commercial manufacturers, such as Winchester, Federal, or Remington, have contracts to the US military for 7.62 ball ammo? If so, it would be interesting to know whether they use the same pressure testing equipment for military and commercial loads and whether they follow the specs on paper for 7.62--50,000 PSI.
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Old February 07, 2001, 13:22   #33
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Winchester and Federal both make and sell .223/.5.56 to the miklitary. I know Winchester does 7.62.

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Old April 06, 2001, 13:29   #34
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Quote:
Originally posted by Scott S:
...I've been looking for some empirical data on the pressure testing method employed by the US military, but I haven't found any, yet. When (if?) I do, that should settle this issue. The closest thing I've "found" is the page from the Army manual that Dano posted last September, stating that the 7.62 is loaded to 50,000 PSI. However, it didn't state what measurement method was used (though I think it was the piezoelectric transducer method, not the copper crusher method).

As for testing at the range, both those methods are beyond my fiscal means. The only thing I can think of is to shoot both 7.62 and .308 loads on the same gas setting and see if the brass is thrown different distances. While that wouldn't tell you what the pressure is, it might give some indication of whether one load is hotter than the other. Or it might not.

I'm open to ideas.
THE MYSTERY IS SOLVED! According to the NRA, the U.S. military uses the copper crusher method to measure pressure. Also, the muzzle velocity for 7.62 NATO is actually about 2808 fps, not 2750 fps as stated above and elsewhere. 2750 fps is the instrumental velocity measured at 78 feet from the muzzle.

Different gas setting requirements for different manufacturers and types of ammo only indicates a difference in port pressure, and may or may not have any relationship to peak chamber pressure.

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Old April 06, 2001, 14:13   #35
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Hey Refinishing Guy, thanks for following up. So, they're actually measuring 50,000 CUP, not PSI. In that case, SAAMI specs and US Mil specs differ by only 2000 CUP, not enough to make any difference to me.

Oh, and good catch about my incorrect supposition regarding the distance brass gets thrown relating to chamber pressure. I don't know what I was thinking when I wrote that.

Touche, Uncle Buck!
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Old April 06, 2001, 15:25   #36
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Quote:
Originally posted by Scott S:
...So, they're actually measuring 50,000 CUP, not PSI. In that case, SAAMI specs and US Mil specs differ by only 2000 CUP, not enough to make any difference to me.
Scott S; Actually, to be 100% technically correct, the pressure for NATO ammo should be expressed as "50,000 p.s.i., as measured by the c.u.p. method". C.u.p. is a p.s.i. measurement, but it measures pressure differently (static) than a strain gauge (dynamic). Both systems measure p.s.i. accurately, but the numbers are different because they measure pressure in a different manner.

Bottom line; .308 Winchester commercial ammo uses thinner cases, is loaded to about 5% greater pressure, and gives about 40 f.p.s. higher muzzle velocity than 7.62 NATO. 7.62 NATO ammo is generally held to slightly smaller dimensional tolerances than .308 Winchster. Other than this, the two cartridges can be considered identical when both are loaded with 150 grain (approx.) bullets. The real differences between the two caliber designations is the headspacing of the chambers of the rifles in which they will be fired. SAAMI headspace spec is generally regarded as between 1.630" and 1.634", which also works perfectly with NATO ammo. NATO specs are much looser, and the maximum Field Reject limit is a whopping 1.6455"!!! With the thick brass used in NATO cases this is perfectly safe, but it would be extremely dangerous to fire .308 Winchester commercial ammo in such a rifle or machine gun. As long as your rifle is headspaced to the middle of SAAMI specifications (approximately 1.632"), you can safely shoot either .308 Winchester OR 7.62 NATO in it.

You might want to check out http://www.fnfal.com/forums/Forum3/HTML/001657.html

Refinishing Guy.

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Old August 27, 2001, 22:24   #37
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Touche?

OK, one more time: The Units of Pressure (the UP in CUP) in the US are Pounds per Square Inch. 50,000 CUP means 50,000 PSI as measured by the copper crusher gauge.

Any pressure units can be readily converted to any other unit of pressure simply by multiplying by a constant. Examples would be converting Bar to PSI by multiplying by 14.5. This is exactly the same as any units of speed can be converted to any other units of speed. You can easily convert FPS to MPH or meters per second by multiplying by a constant.

Since the copper gauge and the piezo gauge both give pressures in the same units (PSI!) no conversion is necessary, but the comparison of the pressure values is invalid as the gauges don't give the same reading.
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Old August 28, 2001, 14:19   #38
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Yep, touche Uncle Buck. Thanks for remaining patient with me while I ranted about which I know not. I think I *finally* understand. Thanks, too, to the Refinishing Guy.

Seems damned foolish to label a pressure CUP when it's really PSI. If it's the measuring method that's different (and not the units of measure) then why don't manufacturers state "XX,XXX PSI as measured by the copper crusher/lead crusher/transducer method" when referring to the pressure of their ammo? Grrrrr.

It's like trying to determine why half the bolts on my Chevy are metric and the other half standard--utter futility.
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Old August 28, 2001, 16:52   #39
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What is foolish or irrational is that the transducer measurements are not labeled "Piezo Units of Pressure" or "Transducer units of Pressure". If they were labeled PUP or TUP, then there wouldn't be all this confusion with CUP.
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Old November 08, 2001, 11:59   #40
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Old November 08, 2001, 14:24   #41
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I did a velocity test on Hirtenber,Radway Green and South African. Those numbers are under "Ammo" "velocities". You guy are scarin the hell out of me. I'll just do the chrono thing.
Seems I have read here that other guy have shot Winchester and Remington .308 through their FAL's and I don't remember any problems being reported.
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Old November 08, 2001, 22:11   #42
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Weeeelll, if you look on page 404 of the A-Square reloading manual Art Alphin lists 150 grain loads in PSI and 180 grain loads in CUP. Oh well. At 51,900 psi with 46 grains of IMR-4064 velocity is 2802 fps. Whaddaya know.

I have reloaded using velocity as the main determinant. A given pressure will return a given velocity up to a point. By loading under, then working up, you reach a listed max velocity load in the books. Don't go over. I have found many times I would reach a listed max before I came near a listed load weight of powder. This is simply due to variations in barrels.
Speaking only for myself, when I find a load listed in CUP or PSI I work out a proportion and only for that cartridge. 50,000 CUP and say 60,000 PSI gives me 1.2. Divide the PSI by this or multiply the CUP by this. It is an easy rule of thumb and only that. I use it to get into the ball park and ALWAYS use my chronograph. Chronys can be had for $89.00 on sale, there is no excuse not to get one. I you want a definitive answer you can get an Oehler set up that'll run around a $1,000 but it is wth the pressure transducer system so you can set it up for your rifles.

"According to the March 2001 issue of Guns & Ammo magazine, and quoting from page 26, regarding the .308 Winchester; "Chamber pressures are 52,000 cup by the copper crusher method and 62,000 psi using the transducer method. That's up slightly from the values for the .30-06, but a little below maximum magnum pressures". Gunthings.

110% correct, the reason it is loaded a little hotter that the '06 is 'cause the Army wanted to maintain the same ballistics as the '06 but in the shorter package to use in the M-14. Pic o page 401 of A-Square manual shows en bloc clip of '06 next to open port of M-14 and the .308 (or Ordnance T-65 as it was called) allowed for a 1 inch shorter receiver.

[ November 08, 2001: Message edited by: gman ]
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Old November 09, 2001, 09:02   #43
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After doing considerable research into this topic I have found only one salient difference between .308W and NATO 7.62. Commercial .308W cases are cheaply made with thin walls to lower cost of production. Nato cases are heavier walled to accommodate "generous" chambers. The vast majority of .308W is loaded to app. 47k and Nato is constrained to a "max" of 50k whereas actual loading avg. at 47k. Bottom line is do not use .308W in "generous" military chambers due to case failure caused by not enough brass to flow and seal. Gar
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Old March 19, 2003, 15:55   #44
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Greetings,

Excellent discussion. I am new at this, so please don't throw too many rocks. Could you do a rough cross check by working backwards? What I mean is, you know the powder load, loaded case volume and powder characteristics for bullets of equivilent weights for both .308 and 7.62x51. If these different cartridges are fired from the same gun and chronographed, would the muzzle velocities be close? Would the .308 be slightly faster? If they are close, with everything else known about the other charachteristics, would the pressures be close regardless what units are used? Does what I am saying make sense? Will Hortense run off with Roger? Tune in next week.... Well, you get the idea. Thanks for listening.

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Old March 19, 2003, 16:49   #45
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7.62x51mm NATO or 308 Winchester?
An Armourer Explains the Differences
Do you have a military surplus rifle chambered in 7.62 x 51mm? If so, I'll bet part of the reason you bought it was to get a "308 Winchester" at a really low price. Well, you screwed up. It's like looking at a Northern Pike and a Muskie. They're close yes, but each is distinctive, and you have to understand what you're looking at.



--------------------------------------------------------------------------------

Early in my military career, I worked on the FNC1 rifle (7.62 NATO). For someone that had previously handled and fired only shotguns and hunting rifles, this was a real treat! Maybe being an armourer wasn't going to be that bad! It was only natural then, that curiousity would generate questions and comparisons about what I thought was the same cartridge - 308 Winchester and the 7.62x51mm NATO.
As part of my personal battery, I had a Midland 2100 chambered for 308 Winchester. This rifle was made in England by a small company - Midland Rifle Co I believe - but owned by Parker Hale. Having lots of military 7.62x51mm rounds available, I shot them from my 2100 from time to time. There was never a problem chambering or firing the military stuff. It was a pleasant diversion over the long (read boring) periods spent at the range for base small arms qualifications.
(I traded it off some years ago for a Smith & Wesson Model 686. It's the only rifle I ever regret losing! - Steve)

Around this period I discovered that shooting reloaded cases fired from an FN was virtually impossible. They didn't want to chamber. My frustration led me to ask an older armourer what was wrong. In a nutshell, he told me that they probably didn't fit because they stretched. I was using a Lee Loader in 308 Winchester and didn't know that they only resized the neck, leaving the rest of the case untouched. The shoulders were blown forward on initial firing, so the case was simply too long to fit my rifle's chamber! Hmmm...
I tried the gauges from work in my own rifle. What a shock! None of them would fit! The corporal brought a set in from home and explained the difference. Here's what I discovered.

308 Winchester (SAAMI) Gauges Shop (Military) Gauges
GO - 1.6300" GO - 1.6350"
NOGO - 1.6340" NOGO - 1.6405"
FIELD - 1.6380" FIELD - 1.6455"

The gauge sets were different too! What gives? He told me that the dimensions of the two new and unfired cartridges were basically the same. The difference lay in two areas - chamber size and cartridge makeup.

Chamber Size
Look at the picture below. The top chamber represents a military rifle, the bottom one a commercial sporter. With many military rifles, their chambers can be significantly longer than, say, a Remington 700. Note that the military chamber would fail a NO GO check, but pass a FIELD check using the proper shop (military) gauges.

There is a .013" difference in chamber length however, between these two "Safe & Serviceable" rifles!


There will be distinct differences between chambers of military rifles based on:

Type
Ishapur, Garand, FN etc.
Amount of Use
Condition at Time of Sale
Just by looking at the dimensions, you can see that using SAAMI gauges is bound to reveal "problems".
Cartridge Makeup
I said at the beginning that these two cartridges were basically the same, dimensionally. While they may share similar external measurements, there is one major difference that you can't see. Case wall thickness.


Military brass is thicker. It needs to be. It was made to function and stretch in a wide variety of firearm chambers. Don't forget that the condition of spent brass is of no importance to the service. Like 99% of military brass, it won't be reused.
Where does that leave you? Follow this rule and you won't have any problems.

DON'T USE YOUR RIFLE IF IT FAILS THE SAAMI FIELD CHECK!!

Why not? Right away you know that the chamber is measuring more than 1.6380" (SAAMI FIELD) - possibly more. It will stretch by a minimum of .008" on first use! Probably more. It may even rupture! You must get the headspace corrected to bring it in line with SAAMI specs. Hey, it's for your own safety! Based on this rule, find your situation below.

I don't reload and use only milsurp ammunition.
You shouldn't have any problems.

I don't reload but want to use commercial ammunition.
There is a potential problem with brass integrity. Commerical cases are NOT as thick as their military cousins. There is less brass, so if you use them in an original, unmeasured military chamber, there may not be sufficient material to stretch and fill the chamber without rupturing. That's bad!!

I reload.
You know that commercial brass is thinner. An easy way to see this is to weigh a fired case of each type. You'll see that the commercial case is lighter. There may not be enough brass left - after lengthening - for any decent case life. You will only aggrevate the situation by full length resizing (it weakens the case by overworking the brass), loading at or near maximum or both.
If you own a milsurp bolt gun like an Ishapore 2A or 2A1, consider neck sizing the brass for better case life, fit and accuracy.

Some Final Thoughts
Military ammunition is loaded to maximum average pressure 50,000 PSI using the modern piezo transducer method of measurement. This standard is used to ensure better consistency round to round. It is proofed at 67,000PSI. The ammunition can then be used in a wide variety of firearms with no ill effects.
Look for the NATO compatibility mark. It indicates that the cartridge is safe to use in any 7.62x51mm NATO firearm in good repair.


NATO mark found on the outside of ammunition containers and the bottom of each case rim.
Commercial ammunition has a SAAMI/ANSI maximum pressure of 62,000PSI. While not every manufacturer may load it to this level, this is the industry established maximum. This is also measured using the piezo transducer method. The proof cartridge pressure is 83,000 to 89,000 PSI. Note the differences between the military test and operational standard vs the commercial one.

The worst case scenario for shooters of 7.62 NATO rifles is as follows. Using a commercially made, maximum pressure cartridge with long headspace, fired from a weak action (ex. converted 93/95 Mausers)! The weaker action is a possible final piece of the 308 vs 7.62 NATO puzzle.
Perhaps no single element will cause your rifle to fail. Maybe you've used your rifle for a while with no ill effects. Usually a rifle will take some abuse before failing. Like a lot of things in life, Murphy's Law - If something can go wrong, it will - could very well catch up with you.
Personally, I don't like the odds.
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Old October 19, 2003, 20:53   #46
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Has anyone ever heard of a problem (kaboom, ruptured case, stinky smell, bulged brass etc.) that could either probably or possibly, be related to the use of commercial .308 used in a NATO chamber, or for that matter NATO ammo being used in a commercial chamber? Please, lets stick to only factory ammo, no handloads of any kind. And no Indian ammo, that proves nothing. It seems to me that so long as the headspace is in tolerance (near the high limit for commercial) that there really isn't much of a problem.
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Old November 27, 2003, 17:17   #47
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Old November 28, 2003, 04:22   #48
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NATO ammunition "Interchangeability"

Cruffler.com credit:
http://www.cruffler.com/trivia-June01.html

Probably the best tabular data I have seen re: 'differences' and what is really 'NATO'?
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Old December 04, 2003, 21:12   #49
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Good data and disscussion guys....I myself would like to see the "problem" with high pressure .308 go away. NO ONE who has a pressure gun, and a vested interest (ammo mfg. / gun mfg.) has given this a second thought, or issued a warning of any kind. Like you guys say....same bullet / same case / same velocity = same pressure. (given the normal differences that components will make in any "load" recipe).
I know army velocity is a little slower (listed at 78 feet from the muzzle, while commercial list is 'muzzle velocity')
And why worry about Army Pressure listing as PSI??? Of course it is PSI because this is a statement the average GI type will understand. I think myself it a CUP number stated as PSI for clarity. This is an ARMY Tech manual, NOT a SAMMI / Scientfic paper.
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Old January 05, 2004, 09:11   #50
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Another discussion on NATO vs. Win. I have bookmarked for reference.

Fulton Armory
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