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Old December 21, 2008, 06:08 PM   #1
FALPhil
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Please Critique This Article: 308 Win vs 7.62 NATO

I'd like to publish this, since there is so much misinformation regarding this subject.
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Old December 21, 2008, 09:02 PM   #2
wogpotter
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I downloaded a copy. I'll read it fully & let you know.
That way I won't make any hasty reading mistakes.
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Old December 22, 2008, 10:48 AM   #3
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I took the time to read & re-read the piece you authored.
It is very good. Clear, laid out in a logical order & well-referenced.
Any changes or additions I'm suggesting are really minor.

The statment about "308 winchester and 7.26 NATO were completely interchangable." in para. 7 might want to mention standard loadings. I only mention this for the sake of completeness with reference to the "specialty" loads that are new to the arena. I'm thinking of "managed recoil" & "Light magnum" type loads here. Maybe also de-fuze the usual "220 grain bullets with slow powder in a Garand" argument as well, before someone else brings it up. Maybe mentioning that military loads are not in the 200+ grain weight?

In the pressure section, where you discuss the differences in measurment systems. Maybe it's worth mentioning that the copper crusher method is total maximum force exerted only, but the pizeo-type plots the pressure curve as well. This is possibly another origin of apparent discrepancies.

Where the chamber & case dimension differences are mentioned, and a table presented, maybe a case/chamber spec drawing might be added?

The final thought may be in an area you don't want to go to as it's marginally relevant, reloading.
You do mention that reloaders need to allow for the differing internal case capacties between commdercial & military cases. Perhaps the differences in these as a table, or list might be helpfull. I have a direct comparison from a loading manual that gives both sets of load data if that would help.

All very minor suggestions, all in all it's an excellent piece of factual documentation.
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Old December 22, 2008, 05:21 PM   #4
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Thanks. That is excellent feedback. I will incorporate your suggestions. I think I know where I can find some pretty good chamber drawings.

I considered spending more time on reloading, since most of the ammo I shoot is ammo that I loaded, but it kind of strays from the primary purpose of the document, which was to dispel some of the myth around the two cartridges.

Thanks again!
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Old December 22, 2008, 07:51 PM   #5
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PLease, Please put this everywhere when you get it ready to go.
I'm blue in the face form de-bunking this myth over & over again.

I've personally loaded, shot, re-loaded, & re shot .308 Win. & 7.62mm NATO in rifles chambered for both for years & years.

I was actually banned from one website for daring to suggest that they weren't different in any practical sense.

The .308 win was a shilen custom-barreled remington 700.
The 7.62's were an M1a (an early one from Springfield armoury, before the mil-spec parts shortage) and an Imbel barreled & recievered FAL.

If anecdotal evidence will help bolster the case consider me on board.
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Old December 22, 2008, 08:31 PM   #6
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You ever hear of Edward Horton? He has written some similar things. I found out about him after I wrote this. He uses the same reloading manual picture I do.
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Old December 22, 2008, 09:07 PM   #7
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I find it odd, the people who seem to think you can't shoot 308s in a 7.62 or vica versa (same with the 223 v 5.56) don't have much experience except what they read on the internet. I've been shooting both in both guns for 30 Plus years, Not just my guns military guns running the AK NG Marksmanship unit. I have had seen zero problems but I also have been told I was full of poop.

Whats funny, the same people that rant about the headspace differance scorn me when I warn of using case gages to get cases propertly sized for gas guns.

But hay, what do I know.
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Old December 23, 2008, 04:38 PM   #8
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Quote:
You ever hear of Edward Horton?
I assume you don't mean the actor?
Other than that I'm afraid not.
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Old December 23, 2008, 10:26 PM   #9
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Is good job.

What I said .

But said much, much more exhaustively.

Is reaaally good job.

Very best regards,

Walt
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Old January 1, 2009, 09:17 PM   #10
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FALPhil,

Sorry to have missed this earlier. I picked it up today from your link in another post. I read your article with some interest. It reaches much the same conclusion about interchangeability that I have. I've certainly shot lots of match issue M118 and M852 in the past. There are a few caveats and fine points that could be addressed on the practical level, and that you might want to take into consideration if you do an edit?

First, Wogpotters table suggestion: I dug out a tiny Excel File I'd made awhile back. I'll put it up on my file repository so you can download it to use or ignore it as you see fit. It might serve Wogpotter's suggested purpose? You could easily expand on it to include other dimensions if you chose to? As it is, it is very short and was hardly worth bothering Excel with except as a convenient way to view it. Screen capture below:



You described the more compressive NATO EPVAT testing, but perhaps in somewhat overly glowing terms. There is nothing wrong with the protocol itself, AFAIK, but the implementation, except for external dimensions, would seem to be "more honor'd in the breach than the observance", except for the US made ammo. Read this rather distressing piece on disassembled NATO surplus ammo. In fairness, it is surplus, perhaps because of the very problems uncovered, but the differences seem a bit too wide spread for that to explain it all.

It has been my supposition that liability concerns have driven large scale commercial cartridge makers to load ammunition down below SAAMI maximums in most instances. U.S. military ammo seems to be loaded to spec. The US military ammo therefore recoils harder in most instances, flattens primers a bit more and generally leaves the impression that military loads are hotter than commercial loads. Well, in most instances they are, but not because the specifications call for it. That's the point of confusion: max SAAMI spec vs. what you actually buy. Even without measuring, I noticed some time back that military .45 ACP hardball had a tad more pop to it than commercial hardball. Military rounds are loaded to almost 400 ft-lbs, and a quick online survey of specifications claimed by big name commercial makers puts their standard pressure loads at about 350 ft-lbs.

You quoted Karl Kleimenhagen on Bramwell's curve fit. He is quite correct. Some time ago I noticed that Excel, possibly what Bramwell used for his regression, could not achieve anything but a linear least squares fit to his data. I realized that was probably because the data as he presented it in his Varmint Hunter article was random in order and the curve fitting algorithms were looking for pressure data numbers to be monotonic. I reorganized his data and tested to be sure his linear fit came out unchanged, then found two curve fits that returned higher R² test values than his linear fit did. Bramwells linear fit got R²=0.927. I got a power regression that returned R²=0.936, and an exponential fit that returned R²=0.939. A perfect fit would be R²=1, so none are fabulous fits, but the illustration with the formulae is attached, should you want a way to get a little closer?

Back in the late 1980's, when the U.S. started getting ready to host the 1992 Palma match, the Palma Committee got Sierra on board to design the 155 grains SMK bullet, and got Winchester on board to design a new .308 Winchester case with maximum internal capacity. Since case dimensions are only specified externally, that leaves a good bit of wiggle room to get clever. What Winchester came up with was a semi-balloon head design with about three grains more water capacity than standard .308 cases. This case’s weight averaged about 156 grains instead of the more usual 170 grains or so, and was headstamped "PALMA" (should anyone got confused). It is an notable, if meaningless, coincidence that the Winchester Palma case weight wound up close to the Sierra Palma bullet weight. Later, Winchester came to adopt this new design as their standard .308 Winchester case. Why not? It costs them less to make and it works fine. It is, however, unique among commercial .308’s for the head configuration on the inside and for total weight and case capacity. I treat it as a separate animal from other .308 commercial cases for that reason. I use it exclusively for long range, though I find I cull nearly 80% to get some within Lapua uniformity levels. I've also noticed Winchester has carried the semi-balloon head over to some other chamberings of late.

Most of the NATO brass I have (Lake City, IMI) seems to run in the 180-186 grain range, give or take, with IMI being the heavier ones. But if you do the math on the total weight, bullet weight and charge weight numbers for the pulled ammo tabulated in the story on NATO cartridges I linked to above, then subtract five grains for the primer, you'll see France made some cases that were only 162 grains. So, again, the generalizations get difficult to stand behind.

Case brass has a density of 8.53 times that of water, so when case external dimensions match, you can just divide that number into the brass weight difference to learn the water capacity difference. Figure the powder difference needed to match pressure and barrel time will be about 2/3 of the water capacity difference if the case is nearly filled wit powder. Otherwise, use 2/3 of the percentage change in total case water capacity (which means you'll have to measure one; my file repository has an Excel file to help with that which compensates for water density changes with temperature).

A hint for loose chambers. If you have a NATO chamber on the large side and are breaking in unfired new brass for it, you can reduce future casehead separations if you first load the brass as squib loads that fireform the case without applying enough pressure to stick the case to the chamber walls. This causes the case to back up and blow open more at the shoulder than at the pressure ring. I use a 100 grain plinker over 8 grains of Bullseye for this. Once initially fireformed, I keep in mind the rule of thumb that it only takes about 0.002" of shoulder setback to guarantee smooth feeding even in a self-loader, and I just don't try to set brass back farther than that, regardless of what size it came from the chamber. That marries the brass to the chamber, but together with annealing, it keeps cases working a long time.

Nick
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Old January 2, 2009, 12:35 PM   #11
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Nick,

Thanks a bunch! That is excellent information.

I have already sent the article out which will be online soon, (I think) but your comments would be a great addendum.
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Old February 15, 2009, 04:58 PM   #12
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1991 USA Palma Ammo Stuff

Nick says about the 1992 Palma ammo:

"This case’s weight averaged about 156 grains instead of the more usual 170 grains or so, and was headstamped "PALMA" (should anyone got confused)."

As one of the earlier US Palma Team members, I was asked to help develop loads for Sierra's new Palma bullet. Winchester had made some prototype cases (weighing about 166 - 168 grains) but they weren't uniform enough ineck wall thickness. Winchester had to retool/reset their case making dies to get 'em near perfect.

We were asked to use new cases weighing between 165 and 170 grains to do our load development with different powder types and charge weights. I called Western Cartridge Company's plant in Alton, IL, to ask 'em about using dies like the ones making the famous WCC58 cases for the US Olympic Free Rifle Team. The engineer there I talked to said they still have that set of dies preserved and tucked away in a back room but didn't dare use them as those perfect 150-grain cases they made were too thin for liability issues of the 1990's; and lurking lawyers sometimes get called about such things.

Winchester finally got their tooling just right and the cases made for the 1991 preliminary match (Rocky Mountain Palma Match, International Division) and 1992 World Palma Matches were headstamped "PALMA 92." Bob Jensen's crew in Tucson, AZ loaded them on two Dillon 1050's; a few hundred thousand rounds. I shot that ammo in the 1991 RMPM scoring the high 5-day aggregate of those long range matches. Weighed some empty cases and they were all about 169 grains, not 156 as you mention. Same for many hundreds I used afterwords. The load used was 45.3 grains of metered (not weighed) IMR4895 with a Federal 210M primer. This load shot 20 randomly selected rounds into 2.7 inches at 600 yards all rounds fired in 15 minutes.

Winchester made another batch of cases a few years later headstamped "PALMA" without any date shown. Those cases were lighter than the original PALMA 92 ones and were used for the tryouts for the 1996 US Palma teams. Maybe this batch is what Nick's referring to.

Last edited by Bart B.; February 16, 2009 at 07:18 AM.
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Old February 16, 2009, 08:32 AM   #13
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US Military's Mixing 7.62 & .308 in Same Rifle

I just read the article. On a scale of 1 to 10, it's a 12. Maybe a 13 (bad luck to those who think it's trash, outstanding to those who know better). I'll add some spice to enhance it's flavor.

After shooting my first M1 during USN boot camp small arms week back in 1956, I became quite interested in small arms marksmanship. When learning about the 4-year-old cartridge, I wrote Mike Walker at Remington (he championed the team that developed the cartridge) and asked about interchangeability between 7.62 and .308 rounds. He wrote back saying they're the same in rifles but not in most folk's minds because of the military versions having slightly different dimensions in chamber and cartridge than commercial versions. He aslo said that in spite of Remington's success of modifying a .300 Savage case and designing what's probably the most accurate 30 caliber round ever, Winchester trumped 'em in the card game of commercial cartridges 'cause they got the first big contract for ammo.

When I started shooting highpower competition in 1964, I used a Navy converted M1 shooting the 7.62 round. (Note: It wasn't until the 1970's that the Army and Marine armorers could build an M14 to equal the accuracy of those Garands.) I'd heard civilians talk about all those "incompatibilities" between the two versions of this round. When some of them saw me shooting Remington .308 match ammo in my 7.62 Garand, they sometimes asked why I was doing such a dangerous thing. My answer was the same as the USMC and US Army using M14's and the USAF using 7.62 M1's; "It shoots more accurate than M118 match ammo!" Most civilians understood they're interchangeable but about 10% of them told us we were _____ (you fill in this blank with your favorite word for dumb, ignorant and/or stupid people).

A friend of mine who used to be on the USAF team (also the first person to shoot the .308 Win. at the US Nationals) had developed a very accurate load for long range use in 7.62 Garands; M118 new primed case with 44 grains of IMR4320 and a Sierra 190 HPMK seated out to almost touch the lands. It's peak pressure was close to that of a blue pill (proof round). The USN Small Arms Match Conditioning Unit in San Diego made this load which won a lot of long range matches. The Unit (and perhaps the USMC and US Army teams) also made "Mexican Match" ammo by pulling bullets from special lots of 7.62 M80 ball (LC12638 and LC12484) and seating a Sierra 168 in 'em. Both lots used 42 grains of a non-canistered powder, IMR4475, that Lake City used for the 8-round clipped ammo for converted USAF and USN Garands as well as their blue pill proof loads. This also was about the most accurate ammo through 600 yards I've shot in 7.62 Garands. All the services' members shooting this stuff in competition were told not to give any of it to the civilians paying the taxes that supported the military competitors. Peak pressures were close to, it not equal to proof loads.

With the above in mind, I called Lake City Arsenal in the early '70's and talked to an engineer involved with load development and accuracy testing. He said their 7.62 NATO proof loads were essentially the same as the Navy's Mexican Match. The difference was they used a 172-gr. match bullet instead of the 147-gr. ball version in the same case with the same 42-gr. of IMR4475. He said he'd heard about the USN and USAF using "blue pill proof loads" in their Garands and oft times shuddered just thinking about it. But we never had any problems.

At the National Service Rifle Matches held at Camp Perry, all the military teams told the civilians to "never" use their 7.62 NATO match ammo in civilian rifles. There was a "safety" problem. Of course, all the military competitors knew they were completely interchangeable and did so by using commercial .308 match ammo in their M1's and M14's and the knowledgeable civilians used M118 match and M80 ball ammo in their bolt guns with total confidence and safety. It was in the late '60's and early '70's that the phrase "political correctness" became popular and a lot of us military competitors feel this 7.62 vs. .308 ammo controversy was the prime cause.

At the 1971 National Matches as a USN Rifle Team member, I was the first person in the USA to complete the standard course of fire with an M16. In talking with military folks from other services then and a few weeks before at the Interservice Matches in Quantico, VA, we discussed the issues of 7.62 vs. .308 and knew it would occur again in another form; 5.56 vs. .223. A few civilians asked us about that at the '71 Nationals and said the differences were the same as those for 7.62 vs. .308. Most interesting, as I observed later, this 22 caliber issue never grew to the magnitude of the 30 caliber one.

When I was on the 1988 US Palma Team and later US International Teams shooting the .308, I learned that the USA ain't the only country where some folks believe the two rounds are separate in every way and safe use requirements. I doubt the issue will go away peacefully.

Kudos for the article. Knowledgeable people will applaud it. Others (and we know who they are) will condemn it.
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Old February 16, 2009, 10:15 AM   #14
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Bart B

Thanks for the bit of history. I got in the game a little later then you (1977) and enjoy reading about those who came before me. My 30 years shooting highpower leads me to agree with you, even if our voices fall on death ears. I spent some time hanging around the Ammo shop at the AMU, and a lot more in dealing with the NGB MTU folks and enjoy reading about the other side (Navy/Air Force/Marines).

I'm looking forward to more of you post.
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Old February 16, 2009, 01:55 PM   #15
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I should have moved this to Art of the Rifle at the outset.

Moving now.
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Old February 16, 2009, 03:02 PM   #16
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FALPhil,

Nice article, very thorough.

I do see one clarification that needs to be made. The comments about 52,000CUP and 62,000PSI being equal need to be qualified. They may be equal in the .308 but they are not at all equal in the general case.

It is not possible in the general case to make such statements because a PSI measurement is a true pressure measurement while a CUP measurement involves, to some extent, measuring the area under the time vs pressure curve.

It is sometimes possible to come up with reasonably accurate approximations between CUP and PSI that work within a single chambering or within a group of chamberings that operate in similar pressure ranges and have generally similar cases but it is NOT possible in the general case. So it might be corrrect to say that both are the same pressure in the .308 but it is not correct to make the unqualified statement that the two figures are correct.

Dr. Ken Oehler determined this from his successful efforts to derive a model which converted electronic pressure (PSI) measurements to CUP. The results were reported in an Article in Sept 2006 Shooting Times. While he managed to make the conversion, it was only possible if the entire electronically measured pressure curve was analyzed. The two figures simply do NOT contain sufficient information to allow conversion between the two in the general case.

A direct quote from Dr. Oehler:

"...there is no way to predict CUP from a peak PSI reading, and there is no way to predict a peak PSI reading from a CUP reading."


Dr. Oehler research indicated in no uncertain terms that there is no way to come up with an accurate generic conversion algorithm that converts from PSI to CUP unless you take the entire pressure curve into account.

Dr. Oehler made a point of noting that it's not even possible to say accurately that PSI will be higher than CUP. His measurements under carefully controlled conditions showed that it is definitely possible for accurate PSI readings to be lower than accurate CUP numbers. There is no accurate general rule of thumb--there can be no accurate approximation. Peak PSI can NOT be accurately converted to CUP nor can CUP be accurately converted to peak PSI.

Therefore while 52,000CUP may be equal to 62,000PSI IN THE .308, that is not a true statement in the general case. They're not equal like 60mph is equal to 100kph.

Formulas that claim to convert CUP to PSI should be viewed with a jaundiced eye. Oehler's research indicated that such formulas, regardless of complexity, are simply NOT possible. It's not that the formula would simply be less accurate than desired--it's that it has no hope of being correct (or even of being a reasonable approximation) in the general case.

The only way to do a general conversion was to have access to a complete pressure vs. time curve. The single numbers simply can not be converted to the other except, perhaps within a single caliber and even then probably only within a limited range of pressures.

Bramwell's article is interesting but several statements he makes are in error. For example, he asserts that the reason it has been said that a conversion shouldn't be attempted was because "they are talking about the old, incorrect use of the term PSI, not the modern, correct use of PSI from strain gauges and piezoelectric pressure meters". That assertion is obviously mere speculation, with no basis in fact when viewed in light of Dr. Oehler's quoted research results. It's not about incorrect term usage it's about fundamental differences in the two readings.

Second, his comment that correlation automatically implies that an accurate conversion formula (or even a general approximation) exists is also in error. Dr. Oehler readily acknowledged that there was a correlation but at the same time stated in no uncertain terms that there was no possible way to convert accurately between the two numbers in the general case.

Likewise, Bramwell states that measurement error is part of what has led to the confusion. Dr. Oehler had very accurate equipment available and still came to the conclusion that the two measurements could not be accurately converted back & forth.

The bottom line is that Bramwell's entire premise indicates a basic misunderstanding about the difference in the two measurement systems. The reason that they can't be converted is that one (PSI) is a single measurement figure indicating a peak value or an average peak value while the other (CUP) is dependent on the entire area under the pressure vs time curve. It is not measurement error that causes the problem, it is that the two systems work in very different ways.

I would say that Bramwell's approximation is good for, and ONLY for exactly the numbers he poked into his correlation attempt. That is, it's a reasonably decent approximation for converting between the SPECIFIC PSI/CUP numbers he used in his data fit. At best it's a reasonable approximation when applied to generally similar cartridges in the specific pressure range he worked with. Taking it any farther is not at all wise given Dr. Oehler's conclusive results. I would also point out that Bramwell is fairly cautious and is careful to state that his formula is not a general formula but should only be used with certain caveats. Unfortunately it seems that few people read his material carefully enough to take note of the caveats he wisely provides.

Just to put things into perspective, Dr. Oehler gained the cooperation of "one of the ammo industry labs" and worked for "the better part of a year" on the research resulting in his evaluation. In addition, he is universally considered to be an expert in the field.
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Old February 16, 2009, 03:34 PM   #17
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good job

Very informative article. Actually answered my quries on the subject.
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