Doncha just love how certain inaccuracies become fact and refuse to die no matter how many times the truth is spoken. Yes, it's 7.62 NATO vs .308 Winchester AGAIN. Shooting Times trotted out the old fiction about .308 Win being a 62,000 psi cartridge and 7.62 NATO is only 50,000, so has much less pressure. WRONG,WRONG thrice again WRONG.

The 62,000 psi figure for 308 Winchester is the latest SAAMI pressure measured using a piezo pressure transducer. The value for 7.62 NATO comes from an old Copper Units of Pressure (CUP) style measurement, and the answer is indeed 50,000 CUP. It's like saying the distance between two points is 62 miles and somebody says, "No, it's much further, like 100 km". Unfortunately there is no "laws of physics" based PSI to CUP conversion, but empirical data does show a relationship. http://www.shootingsoftware.com/ftp/psicuparticle2.pdf

Shortly, somebody will appear and say "Ah, but it's 50,000 psi CUP". There's no such unit of measure. It's like measuring your inseam in grand pianos per decade. Don't go there.

If you compare .308 Win and 7.62 NATO as specified by the European standards folks at CIP, Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives - Wikipedia, the free encyclopedia, lo and behold they are both rated at the same pressure. C'mon now, you're not really surprised, are you?

Yes, the SAAMI spec for the .308 Win CASE is not the same as that for 7.62 NATO. The latter has thicker walls at the head for use in automatic weapons with generous headspace, so it has a little less capacity than .308 Win. Big deal. Go look at the velocity specs listed by any of the big manufactures and you will see they are usually identical for bullets of the same weight. Does that sound like the extra case capacity is making much difference, or that one runs at 20% more pressure than the other? No, it doesn't work for me, either.

I'll just pretend I understand what you're talking about.I'm still awed by the fact that socks can go on either foot.

Steve, have a cup of tea. Relax in the fact that you are smarter than most gun rag writers and fact checkers and probably more than a few editors! Ivan

Steve,

Could you please review 223 Rem vs. 5.56 Nato for us? I am also a little fuzzy on 308 vs. 30-06.

Thank you

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S&WHF 366

slow down there brother..........ya need to come inside outta that heat

Can .308 and 5.56 be shot from the same barrel, or is that .223 and 7.62? The lower on my AR is marked multi caliber so I figure I'm ok but just wanted to make sure.

They can??? Then why have I been putting a L and R on them all these years? Years ago I had to change my name. The only underwear I could find already had the name Calvin on them. Couldn't find any with my original name...............

Is it safe to shoot .308 +P in my Model 10?

as long as it go s boom and hits the target thats all i care about

308 was a copy of the 7.5 French!

Sent from my SCH-I545 using Tapatalk

Everyone is having fun with this, but this is a serious question.

Why would anyone write a formula "X equals negative Z plus 1.5 Y", instead of "1.5 Y minus Z"?

Look at the formula listed on page 4. PSI = -18000 + 1.5CUP (I rounded the numbers - sue me).

Why not say "PSI = 1.5CUP - 18000"? That's the way I learned to write formulas, waaaaaay back in high school algebra class.

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I always take precautions

Oh yeah, and thanks for posting that article. That was extremely interesting.

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I always take precautions

In the future Steve if you are going to start a thread like this please attach a picture of a bird too. I need something to help when my eyes glaze over. Thank you.

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paws for friendship

OK lets all calm down and take it easy. The next thing to come up will be the age old.

magvclip.jpg

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Psalm 23:4

Yeah ok.... I'm still recovering from the truck tire circumference odometer reading gasoline expense account thread.

That's nothin, how about latex gloves, no matter whether you need only a right or only a left hand glove or both you ALWAYS get what you need when you pull them out of the box. How does it know?

I wish my life was that small.

Steve, you've illustrated another reason that except for checking out Ayoob's articles occasionally I don't bother with the gun rags. They're mostly advertising circulars now anyway. And I don't mean just the undisguised ads.

As for the ribbing, I have a touch of OCD myself...

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Oh well, what the hell.

Annoyed at thread drift

Uhhhhh question: don't the copper crushers used to measure CUP come with taridge (sp) tables that give a conversion to psi for the measured length of the crusher? I'm not an engineer and I didn't stay in a Holiday Express last night but I read that in a couple books several years ago.

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Quando omni flunkus moritati.

All for naught, too, since the guy I was working for never reimbursed me a cent anyway(didn't even offer).

Andy

Everytime I have to explain the 308/7.62 223/5.56 nonsense I feel like my head is going to explode. I wish we could go back to the 80's when nobody gave to bits and there was no such thing as a 77gr 5.56NATO. it was a non-issue until the internet.

LVSteve - thanks for good informative article. A shame so much BS followed it. Interesting question on the .223 VS 5.65 - if you have this comparison, would appreciate seeing what you turn up. Get the feeling the results are similar to the 308 VS 7.65 pressure measurements. Would be good to see all such data on a common base, but we can't seem to go from the old English measurements to Metric which is a better system for calculation. Again thanks for useful data.

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TOMBECK

Who likes exponentiation?!

Try THIS on for size:

piezo = crusher * ( 1 + ( crusher^2.2 )/30000 )

This formula is supposedly good for converting CUP values between 0-60 ksi. Beyond 60 ksi, conversions become progressively more inaccurate. Not that converting CUP to PSI was ever an exact science to begin with...

Does your head hurt yet?

Thanks for the info Steve, but it seems like every time I try to read another article to finally get an accurate answer on the .308 v. 7.62 question, and if it is safe to use one in a firearm marked for the other, it reinforces one thought for me.

Get a 30-06.

Let's see what the equation gives us for 50,000 CUP (50 ksi):

piezo = 50 * (1 + 50**2.2/30000) = 50(1.18222) = 59.111 ksi[/FONT]

So it looks like that gives reasonably close results to the OP's statement that 50,000 CUP is roughly 62,000 piezo.

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You're shy a few manners.

You're a better man than me. I tried testing the formula myself and felt like my head was going to explode when I attempted to solve the decimal exponent.

LVsteve: Thanks for that article. I had always heard that the two measures were not convertible, but felt that was not really accurate. This is great information and I appreciate your posting it.

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Richard Gillespie
FBINA 102

So, can I shoot 308 in my M1A?

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Regards, Ron
USASA 1965/69

Crushing copper pellets is peachy keen fine except we have moved on to the "Digital Age" and the transducer, which measures pressure can be hooked up to measure the peak pressure over time... And that's where things get interesting. The pressure peak may be so transient that the copper does not have "time" to sufficiently deform. And spikes like that can result in failure of components. So copper is out and digital is in. Learn to live with it and love it..

help Help my eyes have glazed over I can't see! Ahhhhhhhhh........Whew now where's that keyboard.

I'm confused: between .308 and 7.62, which is a CTG?

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No good deed goes unpunished.

If you are talking factory ammo, it depends on the pressure curve of the powder. I've shot ammo Remington green box FMJ marked .308 from a FAL with no problems. Red-X marked Winchester does not work well at all as it has too slow a powder. Even the adjustable gas system on the FAL cannot cope.

I feel you are very on target with this. (Pun intended!)
From what I have seen recently, the chrono tells us that NATO 7.62 and a variety of 308 hunting ammo were not far off of each other; within 100-150FPS. As to what I have seen in hit power on the plates, all things being equal, they stack up pretty equally in the amount of penetration, the amount of swin the target does, etc.
Yet, with the rifle we tested the ammo on last, the Winchester XP did not cycle the last round and lock the bolt, but NATO ammo, and cheap Wolf ammo would. But the chrono said the Winchester was a little faster. I put this down to powder burn characteristics. Everything else was identical.

Good point!!

^^^^^^^^
This.

CW

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μολὼν λαβέ

It's been awhile since I researched this, and my mind is still in a blur, but this is what I seem to recall...

The .308 and .223 were civilian versions of their respective military counterparts. Even though their pressures look different, even if measured in the same scale, they are not. The military specifications require the pressure to read at the case mouth instead of the back of the case like SAAMI does. When SAAMI set their specifications it was based off of military ammunition, that's the way SAAMI works.

The difference between rifles marked in the civilian calibers, .308 or .223, and the military calibers isn't the ammunition they shoot, it's a slight difference in the lead of the firearms chamber. The respective ammunition is similar and completely interchangeable.

From my research sources, the the chamber leade/freebore spec for 308 Win and 7.62 NATO is the same, but 7.62 NATO has larger allowable headspace with the shoulder area of the chamber being about 0.013 longer. As stated before, this is a nod towards keeping a hot and dirty weapon running in combat. I have a reference .308 Winchester vs. 7.62x51mm NATO 7.62 NATO ammo is happy in the extra space due to its thicker case web. The same article claims that the CUP/PSI confusion was actually started by the Army, which if true is pretty darned funny.

There is a difference in the leade and free bore between 223 Remington and 5.56 NATO. 5.56 NATO is somewhat longer, so it gets away with using the long 77gr bullet without overpressure. Scroll down this page for the precise details. http://www.frfrogspad.com/ar.htm

After that we get into the Wylde and Noveske chambers which attempt to allow for long bullets and 5.56 NATO compatibility while retaining 223 Remington accuracy.

From all the references I can find .223 Remington and 5.56 NATO cases are identical. The Wikipedia article claims that NATO and US milspec pressures for 5.56 are equivalent and the difference in numbers is due to the US military allowing the use of the SAAMI measurement protocol. I can buy into that to some degree but others do not. Indeed, the latest and greatest Mk 262 77gr rather fast for a round that is supposed to be running at SAAMI equivalent pressures, and this tester even says "NATO pressure". MK262 clone 77Grain 5.56 load problems / updated OCW tests It has been acknowledged that the gas port pressure with this round is way higher than normal. How much of that is "boost" and how much is powder burn rate I cannot say. Special powder you cannot buy at Midway is suspected.

The US armed forces do play fast and loose when it comes to pressures, especially with special weapons like sniper rifles. The latest 300 Win Mag round uses a special brew that is well past SAAMI specs. This was done so that the round would not appear too inferior to the .338 Lapua. They get away with it by using custom built rifles with new cases and load lengths matched to the gun. Even so I feel it would be prudent to put a "useful life" limit on such a tweaked weapon.

I just measured my inseam and it came out to 3 pianos from the 50s.

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Thirty characters. Exactly...

You guys have just given me a belly full of laughs.

I do it regularly. Nothing over 168gr. though as I value my op-rod.

Steve, I'm afraid your links are no more credible than the rag you mentioned in your first post. I didn't read all of them, mostly because there were so many mistakes it wasn't worth continuing.

I don't consider anything on the internet as "fact" unless it's from a reputable source. This isn't either, but I am going to try to verify the information in post nine on the chamber dimensions as it appears your source on the "headspace" issue is pretty wrong.
7.62 x 51 mm and .308 winchester - M14 Forum

The 223/556 argument never ends. I've heard "experts" say it's OK to shoot 556 out of a 223-stamped gun while others say you shouldn't.

The folks at CZ ..... say that their .223 527s are good to go with 5.56.....

I tend to ask the maker.

Springfield said I could use 7.62 or 308 in my M1A.

Ruger said I could use 223 or 5.56 in my Mini 14.

Ruger also said to only use 223 in my #1.

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I always take precautions

I'm not seeing any difference the headspace data in your link and the one I quoted.

As for confusion over pressures, I see much of it on the M14 forum and in that thread. Consider the factors for confusion:

1) NATO/CIP have their test standards and the answers are in PSI.

2) SAAMI have a different test standard and the answers are also in PSI, but will be slightly different from the NATO/CIP values for the same round.

3) Back in the day the US used CUP to judge chamber pressure and it gives readings in the same order of magnitude as PSI measurements with the same ammo, but CUP readings are different enough to cause problems.

4) Add poor attention to detail, lack of the appreciation for scientific units leading to careless mixing of PSI and CUP, and throw in a liberal dose of poor reading comprehension (ain't got no time fo dat ) and guess what you get?

And I too am awed by that, and marvel at the fact that "Tube Socks" need no orientation of the heel to be addressed when
pulling them on!

The .223 vs .556 can get real confusing with its labeling. Some ammo is labeled both (.223/5.56). Which is it? Some just .223, other just 5.56. I've been told some ammo labeled .223 is really 5.56. Is it really?

In any case, I've shot it all in my .223 M70, my dad has shot it in his ruger #1. It has all been fine. I suspect the most important factor is the real heavy bullets may not stabilize due to the slower twist rate. 62 gr is the heaviest I've used.

I have a hard time believing that a m70 or #1 chamber is weaker than an ar15 chamber.

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Common sense isn't so common.

Post of the week!

No differences (okay a very, very slight difference in capacity that won't matter) in the cases. The differences is the military rounds are typically longer than sporting .223s, so 5.56-chambered rifles have a longer throat than sporting .223 rifles. A 5.56 round may cause the bullet to engage the rifling in a .223 chambered rifle, and thus raising pressures. If you chamber a longer 5.56 round and after ejecting it find no rifling marks on the bullet, your throat is probably long enough.

Secondly, military rounds are hotter than sporting .223s. Will sporting rifles handle the higher pressures? Maybe, maybe not.

I've noticed most ARs are compatible with both rounds, but a few are .223 only chambered. Why a box of ammo would be marked .223/5.56 is beyond me and sets the stage for a possible accident.

I would suspect that ammo marked 223/556 is considered by it's maker to be safe to use regardless of the chambering specs and throat dimensions. I have used both interchangeably for years with no difficulty in quite a few different rifles.
Military chambers are often a little generous (to allow for dirty ammo) and may well have a longer throat but I really doubt there is a significant difference between commercial and military loads with the same weight bullets. I can see where a tight, match spec chamber in 223 could have problems with mil spec ammo but it could very well have trouble with some 223 commercial stuff as well. As more than one poster already mentioned, pressure measuring methods vary and it's not easy to relate one system to another.