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7.62 X 51 cartridge vs. .308 Winchester: the differences!

This is a discussion on 7.62 X 51 cartridge vs. .308 Winchester: the differences! within the Ammunition forums, part of the M14 M1A Forum category; I found this informative discussion on the .308 Winchester vs. 7.62 X 51 this morning. This information has been provided on these forum pages in ...


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Old July 3rd, 2011, 11:38 AM   #1
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7.62 X 51 cartridge vs. .308 Winchester: the differences!

I found this informative discussion on the .308 Winchester vs. 7.62 X 51 this morning. This information has been provided on these forum pages in bits and pieces, and may well exist in total somewhere, but I thought perhaps it would be worth a revisit for any newbies or those who may be confused by the differences.

From the www.6mmbr.com website, select the cartridge comparisons and select the .308 info page, and you'll find a good article on the cartridge, it's potential, bullets & loads. The following is a short excerpt:

"Before we go much further, we want to address the oft-posed question “Are the .308 Winchester and 7.62×51 NATO one and the same?” The simple answer is no. There are differences in chamber specs and maximum pressures. The SAMMI/CIP maximum pressure for the .308 Win cartridge is 62,000 psi, while the 7.62×51 max is 50,000 psi. Also, the headspace is slightly different. The .308 Win “Go Gauge” is 1.630″ vs. 1.635″ for the 7.62×51. The .308′s “No-Go” dimension is 1.634″ vs. 1.6405″ for a 7.62×51 “No Go” gauge.

That said, it is normally fine to shoot quality 7.62×51 NATO ammo in a gun chambered for the .308 Winchester (though not all NATO ammo is identical). Clint McKee of Fulton Armory notes: “[N]obody makes 7.62mm (NATO) ammo that isn’t to the .308 ‘headspace’ dimension spec. So 7.62mm ammo fits nicely into .308 chambers, as a rule.” You CAN encounter problems going the other way, however.

NOTE: A commercial .308 Win round can exceed the max rated pressure for the 7.62×51. So, you should avoid putting full-power .308 Win rounds into military surplus rifles that have been designed for 50,000 psi max.

For more information on this interesting topic, read the following articles: Gun Zone’s 30 Caliber FAQ; Cruffler.com Technical Trivia, June 2001; and last, but not least, Steve Redgwell’s .308 vs 7.62×51 Analysis, which really provides a definitive explanation. Reloaders should also note that military ammo often is made with a thicker web. Consequently the case capacity of 7.62×51 brass is usually less than that of commercial .308 brass. You may need to reduce recommended .308 Winchester loads by as much as 2 full grains, if you reload with military 7.62×51 brass, such as Lake City or IMI.


Enjoy, and good shooting!

Thanks from budster, Scorp and DEVILDG03
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Old July 3rd, 2011, 12:02 PM   #2
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62,000 maybe max for commercial .308 Winchester ammo , but I believe industry 'standard' pressure is held to 52,000 or 55,000.

BUT , ya also need to use caution when using some 7.62x51 NATO in certain .308 Win chambered semi-auto rifles too. My DPMS LR-308 came with a warning about the bullet ogive on some surplus ammo might contact the shorter leade in commercial chamberings , especially match chambers , leading to out of battery firings.

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Old July 3rd, 2011, 12:45 PM   #3
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If you look at the way they measure pressure on a 7.62x51 vs the .308 Win via a chart, its apples and oranges. That is, they are not measured in the same manner. SAAMI is measured at the neck and NATO is measured in the wide part of the chamber. This alone can account for a very significant difference, several tens of thousands of pounds, between the two. Measured similarly, the pressure of a 7.62 NATO round is almost identical to that of a .308 Win. round.

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Old July 3rd, 2011, 02:43 PM   #4
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Unfortunate this myth is repeated ....

The difference between the chamber pressures is because the readings were established using two different methods of measurement. It truly is comparing apples to oranges. If the same measurement method is used for both, the difference is minimal.

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Old July 3rd, 2011, 02:52 PM   #5
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This discussion pops up all the time and many have their own view of what is true and what isn't, but just to show how mixed up the data is let me quote the Army's cartridge data for the 7.62mm ammo that is used in the M14.

Army Ammunition Data Sheets, Small Caliber Ammunition, FSC 1305, dated April 1994.

M118
  • Use: Rifles, 7.62mm, M14, M21, M24, and M40A1. The
    cartridge is intended and specifically prepared for use in
    high accuracy weapons.
  • OAL - 2.83"
  • Bullet - 172 grain
  • Powder - IMR 4895 and WC 846
  • Powder Charge Weight - 44 grains
  • Chamber pressure ....... 50,000 psi
  • Velocity ....................... 2640 fps
M852
  • Use: Rifle, 7.62 mm, M14, NATIONAL MATCH. The
    cartridge is intended and specifically prepared for use in
    those weapons designated as competitive rifles and also
    for marksmanship training. The cartridge is not for combat use.
  • OAL - 2.83"
  • Bullet - 168 grain
  • Powder - IMR 4895
  • Powder Charge Weight - 42 grains
  • Chamber pressure ....... 50,000 psi
  • Velocity ....................... 2550 fps
Look at the chamber pressures, the Army says that they are the same. So you take two cartridges that have the same OAL, different bullet weights, different powder charge weights, and different muzzle velocities, but you list their chamber pressures as being the same? Not very likely.

I ran the numbers through my copy of QuickLOAD (internal ballistics software) and these are the chamber pressures and muzzle velocities that the software predicts.

M118 (using IMR 4895)
  • Chamber Pressure - 69372 PSI
  • Muzzle Velocity - 2778 FPS
M852
  • Chamber Pressure - 49088 PSI
  • Muzzle Velocity - 2575 FPS
Now some will dismiss my numbers because they were calculated by computer software, but notice how close the numbers are for the M852 cartridge. In addition I know from personal experience that my field tests give me numbers that are pretty close to what the software predicts. Regardless, the Army says that they get the same pressure with these two different bullet weights and muzzle velocities and I'm sure that most of us will agree that that's pretty much impossible. On the other hand, if you accept that QuickLOAD predicted the M852 numbers accurately, then you have to accept that the M118 numbers produced by QuickLOAD are pretty close to what you would find in field tests. In which case, the M118 cartridge is producing chamber pressures upwards of 70000 PSI.

A lot of people will call BS on that chamber pressure because they have been told that the M14 platform was designed to withstand pressures of no more than 50000 PSI. Well think about the following;
  • SAAMI standards specify that the proof load for a .308 Win produce 74500 PSI
  • The M1A, as manufactured by Springfield Armory, must meet the SAAMI standards for firearms due to liability issues.
  • Therefor, the M1A is proof tested up to 74500 PSI.

Taking that info in to consideration, it would not be impossible then for the pressure of the M118 cartridge to be as high as QuickLOAD predicts.

In addition, the military mislabeled the units of measure for chamber pressure for many years (see the conversation at this link

7.62x51... the real facts!

They used the term PSI when in fact they were using the CUP testing method, that method results in CUP units not PSI. CUP values are always lower than actual PSI values. I have not been able to find updated chamber pressure data since they changed their testing process to using a pressure sensor (true PSI values).

As a point of interest, I checked the numbers for the military's standard M80 ball ammo. QuickLOAD does not have a listing for the powder (WC 846) or bullet (146 grain FMJ) used in the military cartridge so I used the same powder charge weight (46 grains) with BL-C2 which is reputed to be a close match and a Winchester 147 grain FMJ bullet. The resulting numbers are;

M80 Ball equivalent
  • Chamber Pressure
    Army - 50000 PSI
    QuickLOAD - 53490 PSI
  • Muzzle Velocity
    Army - 2750 FPS 78 feet from the muzzle (they misprinted the speed and listed it as 27501 FPS, I assume it should be 2750.1 FPS)
    QuickLOAD - 2798 FPS (QuickLOAD predicts the actual velocity at the muzzle, not 78 feet away and that, I'm sure, explains the speed difference)
As for what my preferences are, I don't like to use loads that produce anything over 54000 PSI.

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Old July 4th, 2011, 02:42 PM   #6
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Great stuff, RAMMAC! I sure wish that I could run that QuikLoad Ballistic software on my Mac. And they tell us that MACs are Sooooo much more sophisticated (well, yeah, they are, but why no ballistic software then?).

Anyhow, yes, that info on chamber pressures seems to be bunk as regards commercial .308 ammo running 62k psi. Wow! That's right up there with Weatherby et al @ 65k, and the MkV Wby action is hugely stronger than the M14's!

I suspect a massive and significant variation in the measurement standards used, possibly even across different teams in the same military ballistics lab, or in it's interpretation by Staff gunnery Sgt. Smith vs SSgt. Jones! Not to mention the calibration of the test equipment and so on. Otherwise, that first 20 rounds I ran through my rifle, Federal Premium .308, in 180 gr, would have popped the whole caboodle wide open!

Interesting that there's similar such listed diffs. in the .223, 5.56 and .223 Wylde. In theory, all work in each other's chambers, but dimensions & pressures are all different.

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Old July 4th, 2011, 03:12 PM   #7
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Great stuff, RAMMAC! I sure wish that I could run that QuikLoad Ballistic software on my Mac. And they tell us that MACs are Sooooo much more sophisticated (well, yeah, they are, but why no ballistic software then?).

Anyhow, yes, that info on chamber pressures seems to be bunk as regards commercial .308 ammo running 62k psi. Wow! That's right up there with Weatherby et al @ 65k, and the MkV Wby action is hugely stronger than the M14's!

I suspect a massive and significant variation in the measurement standards used, possibly even across different teams in the same military ballistics lab, or in it's interpretation by Staff gunnery Sgt. Smith vs SSgt. Jones! Not to mention the calibration of the test equipment and so on. Otherwise, that first 20 rounds I ran through my rifle, Federal Premium .308, in 180 gr, would have popped the whole caboodle wide open!

Interesting that there's similar such listed diffs. in the .223, 5.56 and .223 Wylde. In theory, all work in each other's chambers, but dimensions & pressures are all different.

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Old July 5th, 2011, 10:07 AM   #8
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Oh my gosh, here we go again.......

The psi figures "quoted" are apples and oranges. The 62,000 psi is the current figure for SAAMI based on piezo transducers pressures. The 50,000 psi quoted for the 7.62 NATO is based on the C.U.P. (Copper Unit Pressure) measuring systems. Those are total different and a comparison is totally meaningless.

The actual psi maximum average working pressures (MAPs or PMAPs) for both the 7.62 NATO and the .308W are pretty close to the same; 50,000 C.U.P. or 62,000 psi. The military is now also using a case mouth transducer to measure the psi of 7.62 NATO and 5.56 NATO BTW. I have been measuring the pressure of various lots of US and foreign 7.62 NATO and commercial .308W ammunition in 3 test rifles using an Oehler M43 for several years now. I can show you 7.62 NATO ammunition that is higher that .308W ammunition and visa versa all tested in the same test rifle on the same day under the same conditions. Point is both cartridges are close to the same and it just depends on the specific type and lot of ammunition as to which will have a higher psi. Also don't assume that because the MAP is listed at 62,000 psi that all such ammunition is loaded to that....it isn't. I've yet to find a commercial .308W or a 7.62 NATO MAP that hits that. A couple of each have hit 60,000 psi but most quality .308W/7.62 NATO that meets spec is in the 56 -58,000 psi range. The loading criteria is based on a velocity specification (+/- a lot more fps than you would believe) that stays within SAAMI psi specs (more than just the MAP BTW) and gives adequate "accuracy".

The measured "difference" between the two cartridges is most often in the velocity. The .308W many times will have a higher velocity for a given bullet weight (say 150 vs 147) than the 7.62 NATO. Many assume the higher velocity means a higher psi. That is not the case most often. It means a slower powder was used in the .308W as the time/pressure curves demonstrate. The cartridge case external specifications are the same also. Ever try to buy a set of "7.62 NATO" loading dies? No, probably because they are all .308W dies because the two cartridges are the same externally. Yes milsurp chambers are more generous (need to be to ensure reliable functioning) than commercial .308W but comparing chambers and concluding the cartridges are there for different is an erroneous conclusion.

With bullets of 150 - 165 gr in the .308W there isn't enough of a slower time pressure to harm the M1A/M14. There is just an increase of 75 -100 fps or so velocity. The gas port psi is about the same with those .308W loads as with M118SB or M118LR ammunition.

Newer .308W ammunition of "super performance" or "extended range" have specially blended and much slower powders and should not be used in gas guns. This because of a much slower time/pressure curve that puts too much psi at the gas port. It is not because of increased MAPs in the .308W. Those types of ammunition have MAPs that are like any other .308W commercial ammunition that are within SAAMI specs.

Bottom line is; before going to different sites and cross referencing the "data" one should be aware of how the "data" was obtained. Most often, in the case of .308W vs 7.62 NATO, the "data" was obtained by totally different methods and is meaningless in comparison.


Larry Gibson


Last edited by Larry Gibson; July 5th, 2011 at 10:24 AM.
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Old July 5th, 2011, 11:18 AM   #9
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Great info Larry, I don't know if you saw it but I linked to your information at

7.62x51... the real facts!

And just for clarification, do you have any real data on the M118 sniper round?



Quote:
Originally Posted by Larry Gibson View Post
Oh my gosh, here we go again.......

The psi figures "quoted" are apples and oranges. The 62,000 psi is the current figure for SAAMI based on piezo transducers pressures. The 50,000 psi quoted for the 7.62 NATO is based on the C.U.P. (Copper Unit Pressure) measuring systems. Those are total different and a comparison is totally meaningless.

The actual psi maximum average working pressures (MAPs or PMAPs) for both the 7.62 NATO and the .308W are pretty close to the same; 50,000 C.U.P. or 62,000 psi. The military is now also using a case mouth transducer to measure the psi of 7.62 NATO and 5.56 NATO BTW. I have been measuring the pressure of various lots of US and foreign 7.62 NATO and commercial .308W ammunition in 3 test rifles using an Oehler M43 for several years now. I can show you 7.62 NATO ammunition that is higher that .308W ammunition and visa versa all tested in the same test rifle on the same day under the same conditions. Point is both cartridges are close to the same and it just depends on the specific type and lot of ammunition as to which will have a higher psi. Also don't assume that because the MAP is listed at 62,000 psi that all such ammunition is loaded to that....it isn't. I've yet to find a commercial .308W or a 7.62 NATO MAP that hits that. A couple of each have hit 60,000 psi but most quality .308W/7.62 NATO that meets spec is in the 56 -58,000 psi range. The loading criteria is based on a velocity specification (+/- a lot more fps than you would believe) that stays within SAAMI psi specs (more than just the MAP BTW) and gives adequate "accuracy".

The measured "difference" between the two cartridges is most often in the velocity. The .308W many times will have a higher velocity for a given bullet weight (say 150 vs 147) than the 7.62 NATO. Many assume the higher velocity means a higher psi. That is not the case most often. It means a slower powder was used in the .308W as the time/pressure curves demonstrate. The cartridge case external specifications are the same also. Ever try to buy a set of "7.62 NATO" loading dies? No, probably because they are all .308W dies because the two cartridges are the same externally. Yes milsurp chambers are more generous (need to be to ensure reliable functioning) than commercial .308W but comparing chambers and concluding the cartridges are there for different is an erroneous conclusion.

With bullets of 150 - 165 gr in the .308W there isn't enough of a slower time pressure to harm the M1A/M14. There is just an increase of 75 -100 fps or so velocity. The gas port psi is about the same with those .308W loads as with M118SB or M118LR ammunition.

Newer .308W ammunition of "super performance" or "extended range" have specially blended and much slower powders and should not be used in gas guns. This because of a much slower time/pressure curve that puts too much psi at the gas port. It is not because of increased MAPs in the .308W. Those types of ammunition have MAPs that are like any other .308W commercial ammunition that are within SAAMI specs.

Bottom line is; before going to different sites and cross referencing the "data" one should be aware of how the "data" was obtained. Most often, in the case of .308W vs 7.62 NATO, the "data" was obtained by totally different methods and is meaningless in comparison.


Larry Gibson

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Old July 6th, 2011, 09:14 AM   #10
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As I stated previously it varies lot to lot. Test results here are with a 24" barrel with 10" twist. Additional testing in 12 and 14" twist barrels generally results in respectively lower psi while velocities stay pretty much the same. The time/pressure curve is slower as the twist decreases. Note also

Three lots of of M118 White Box run between 56,400 and 59,700 psi(M43).

5 lots of M118SB ran 56,700 to 61,700 psi(M43)
Note; one lot of LC91 (6th) ran 63,300 psi(M43) and produced 125+ fps over the other 5 lots. Accuracy was also very poor.

4 lots of M118LR (one with ball powder) ran 57,900 to 61,200 psi(M43).

Additionally for comparison in this thread to demonstrate the fact that a statement such as "the .308W is higher pressure loaded than 7.62 NATO" is not a correct statement.;

Winchester 150 gr PP: 55,200 psi(M43)
Remington 150 gr CL: 56,100 psi(M43)
Federal 150gr PS: 55,500 psi(M43)
Winchester 180 gr ST; 56,900 psi(43)

BTW
Numerous lots of US and foreign M80 7.62 NATO measured psi fall above and below most factory .308W ammunition. Also as I previously mentioned some lots of .308W factory ammunition are right up close to the MAP of 62,000 psi with M118.

Additional M80 (US and foreign) tested on the same day, consecutively, and under the same conditions;

IVI70: 53,900 psi(M43)
LC74; 52,100 psi(M43)
WRA69: 60,400 psi(M43)
LC74 (steel jacketed bullets); 58,200 psi(M43)
LC87; 64,800 psiM43) (yup, that one is right up there and is the highest 7.62 NATO or factory .308W I have recorded)
LC90; 59,700 psi(M43)
IVI70; 58,000 psi(M43)
Cavim 91; 53,100 psi(M43)
Chinese 61 92; 45,700 psi(M43) (100+ fps less than US M80)
OFV82; 42,600 psi(M43) (200+ fps less than US M80)
MAL 11-83; 52,200 psi(M43)
Wolf M80; 45,800 psi(M43) (100+ fps less than US M80)
FNM 80-49; 50,900 psi(M43)

As you can plainly see there is no "rule" as to which cartridge (.308W or 7.62 NATO) has the higher psi. You just puts your money down and takes what you get.........


Larry Gibson


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Old July 6th, 2011, 10:32 AM   #11
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That's interesting. Based on your numbers, QuickLOAD's estimates are very close, if I base the calculations on BL-C2 rather than IMR 4895. BL-C2 gets me very close to your pressure and the military's published muzzle velocity.

M118
Use: Rifles, 7.62mm, M14, M21, M24, and M40A1. The
cartridge is intended and specifically prepared for use in
high accuracy weapons.
  • OAL - 2.83"
  • Bullet - 172 grain
  • Powder - IMR 4895 and WC 846
  • Powder Charge Weight - 44 grains
  • Chamber pressure ....... 50,000 psi
  • Velocity ....................... 2640 fps

QuickLOAD results using 44.0 grains of IMR 4895
Chamber Pressure - 69,372 PSI
Muzzle Velocity - 2778 FPS

QuickLOAD results using 44.0 grains of BL-C2 (closest equivelent to WC 846)
Chamber Pressure - 60,024 PSI
Muzzle Velocity - 2664 FPS






Quote:
Originally Posted by Larry Gibson View Post
As I stated previously it varies lot to lot. Test results here are with a 24" barrel with 10" twist. Additional testing in 12 and 14" twist barrels generally results in respectively lower psi while velocities stay pretty much the same. The time/pressure curve is slower as the twist decreases. Note also

Three lots of of M118 White Box run between 56,400 and 59,700 psi(M43).

5 lots of M118SB ran 56,700 to 61,700 psi(M43)
Note; one lot of LC91 (6th) ran 63,300 psi(M43) and produced 125+ fps over the other 5 lots. Accuracy was also very poor.

4 lots of M118LR (one with ball powder) ran 57,900 to 61,200 psi(M43).

Additionally for comparison in this thread to demonstrate the fact that a statement such as "the .308W is higher pressure loaded than 7.62 NATO" is not a correct statement.;

Winchester 150 gr PP: 55,200 psi(M43)
Remington 150 gr CL: 56,100 psi(M43)
Federal 150gr PS: 55,500 psi(M43)
Winchester 180 gr ST; 56,900 psi(43)

BTW
Numerous lots of US and foreign M80 7.62 NATO measured psi fall above and below most factory .308W ammunition. Also as I previously mentioned some lots of .308W factory ammunition are right up close to the MAP of 62,000 psi with M118.

Additional M80 (US and foreign) tested on the same day, consecutively, and under the same conditions;

IVI70: 53,900 psi(M43)
LC74; 52,100 psi(M43)
WRA69: 60,400 psi(M43)
LC74 (steel jacketed bullets); 58,200 psi(M43)
LC87; 64,800 psiM43) (yup, that one is right up there and is the highest 7.62 NATO or factory .308W I have recorded)
LC90; 59,700 psi(M43)
IVI70; 58,000 psi(M43)
Cavim 91; 53,100 psi(M43)
Chinese 61 92; 45,700 psi(M43) (100+ fps less than US M80)
OFV82; 42,600 psi(M43) (200+ fps less than US M80)
MAL 11-83; 52,200 psi(M43)
Wolf M80; 45,800 psi(M43) (100+ fps less than US M80)
FNM 80-49; 50,900 psi(M43)

As you can plainly see there is no "rule" as to which cartridge (.308W or 7.62 NATO) has the higher psi. You just puts your money down and takes what you get.........


Larry Gibson


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Old July 6th, 2011, 10:55 AM   #12
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So it seems that, in general, slightly higher pressures are created by U.S. made commercial ammo but only by a couple of thousand PSI (rough averages based on Larry's testing; 55,600 PSI commercial vs. 53,600 PSI military surplus). But, the surplus ammo swings wildly (a range of at least 22,200 PSI with the tested ball ammo) while the commercially equivalent ammo is pretty consistent and probably doesn't vary by more than a 1000 PSI within any single bullet weight/load combination.

So after all is said and done it means that the vast majority of the ammo used in these rifles exceeds the claimed 50,000 PSI max pressure by at least 3,000 PSI in most cases, unless you hand load all of your ammunition. And even then we don't know exactly what the pressures are, I wish I could test some of my hand loads to see if they are producing the pressures that I think they are. But I will say this, this data makes me feel more confident in the numbers that QuickLOAD is producing.

garbage in = garbage out.

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Old July 6th, 2011, 09:42 PM   #13
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So it seems that, in general, slightly higher pressures are created by U.S. made commercial ammo but only by a couple of thousand PSI (rough averages based on Larry's testing; 55,600 PSI commercial vs. 53,600 PSI military surplus). But, the surplus ammo swings wildly (a range of at least 22,200 PSI with the tested ball ammo) while the commercially equivalent ammo is pretty consistent and probably doesn't vary by more than a 1000 PSI within any single bullet weight/load combination.

I don't think it's valid to average the velocities given for a comparison. I only list a small sample of .308W to demonstrate a point. As mentioned I can show you commercial .308W ammunition that is right up ther at 61,000+ psi. I can also show you some that is lower psi than the listed examples. The point was that some of it falls in the 56 -58,000 psi range which is lower than the M118 examples given. Also the psi extreme spread for the 10 shot test strings actually varied varied from 3,300 psi to 7, 900 psi, not the 1,000 psi you're assuming.

So after all is said and done it means that the vast majority of the ammo used in these rifles exceeds the claimed 50,000 PSI max pressure by at least 3,000 PSI in most cases, unless you hand load all of your ammunition.

Unfortunately your using C.U.P. psi figures and comparing them to the psi obtained by electronic means (piezo transducer or strain gauge). The two are not the same. The "50,000 psi" you refer to is the C.U.P. method using copper cruchers. The MAP (Maximum Average Pressure) for the .308W/7.62 NATO is 62,000 psi as measured by piezo transducers or strain gauges.

garbage in = garbage out.

In this case it is garbage out only if we mistakenly compare garbage in C.U.P. derived psi's with piezo transducer/strain gauge derived psi's. Use the correct figure of 62,000 psi for the comparison of the data I gave and you'll see that all but 2 loads were under that listed MAP.

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Old July 6th, 2011, 09:50 PM   #14
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Quote:
Originally Posted by Larry Gibson View Post
RAMMAC

So it seems that, in general, slightly higher pressures are created by U.S. made commercial ammo but only by a couple of thousand PSI (rough averages based on Larry's testing; 55,600 PSI commercial vs. 53,600 PSI military surplus). But, the surplus ammo swings wildly (a range of at least 22,200 PSI with the tested ball ammo) while the commercially equivalent ammo is pretty consistent and probably doesn't vary by more than a 1000 PSI within any single bullet weight/load combination.

I don't think it's valid to average the velocities given for a comparison. I only list a small sample of .308W to demonstrate a point. As mentioned I can show you commercial .308W ammunition that is right up ther at 61,000+ psi. I can also show you some that is lower psi than the listed examples. The point was that some of it falls in the 56 -58,000 psi range which is lower than the M118 examples given. Also the psi extreme spread for the 10 shot test strings actually varied varied from 3,300 psi to 7, 900 psi, not the 1,000 psi you're assuming.

So after all is said and done it means that the vast majority of the ammo used in these rifles exceeds the claimed 50,000 PSI max pressure by at least 3,000 PSI in most cases, unless you hand load all of your ammunition.

Unfortunately your using C.U.P. psi figures and comparing them to the psi obtained by electronic means (piezo transducer or strain gauge). The two are not the same. The "50,000 psi" you refer to is the C.U.P. method using copper cruchers. The MAP (Maximum Average Pressure) for the .308W/7.62 NATO is 62,000 psi as measured by piezo transducers or strain gauges.

garbage in = garbage out.

In this case it is garbage out only if we mistakenly compare garbage in C.U.P. derived psi's with piezo transducer/strain gauge derived psi's. Use the correct figure of 62,000 psi for the comparison of the data I gave and you'll see that all but 2 loads were under that listed MAP.

Larry Gibson
That must be why the The American Rifleman magazine posted a small article a couple of years ago that they had found batchs of MilSurp 7.62NATO ammo that had higher pressures than Commercially made .308Win ammo.


Last edited by Sailormilan2; July 7th, 2011 at 06:34 AM.
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Old July 7th, 2011, 12:19 AM   #15
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First of all Larry, I wasn't trying to be scientifically exact, I was trying to summarize your data so that I could try to keep all this information in perspective. Secondly, I'd like to clear up some misconceptions you have about what I posted.

Quote:
Originally Posted by Larry Gibson View Post
I don't think it's valid to average the velocities given for a comparison. I only list a small sample of .308W to demonstrate a point. As mentioned I can show you commercial .308W ammunition that is right up ther at 61,000+ psi. I can also show you some that is lower psi than the listed examples. The point was that some of it falls in the 56 -58,000 psi range which is lower than the M118 examples given. Also the psi extreme spread for the 10 shot test strings actually varied varied from 3,300 psi to 7, 900 psi, not the 1,000 psi you're assuming.
I know that statistically you didn't provide a large enough sample to prove much of anything, but I thought that you posted this info as an example of what you commonly see as the pressures associated with the commercial 150 grain bullet loads. I know that you have done a lot pressure testing and because of that I assumed more than I should have.

Quote:
Winchester 150 gr PP: 55,200 psi(M43)
Remington 150 gr CL: 56,100 psi(M43)
Federal 150gr PS: 55,500 psi(M43)
The variance is roughly 1,000 PSI between the velocities you listed for these bullet weights. I wouldn't know about any other pressure variations since you didn't post that data. But if your only point was to show that some commercially equivalent cartridges have lower pressures than the military version then you succeeded and I now understand. My only real conclusion from the pressure data that you posted was that commercial ammo seems to have more consistent pressures than military ammo, regardless of the actual values. Maybe that isn't scientifically proven with your small sample but from your experience, wouldn't you say that it was a reasonable claim?

Quote:
Originally Posted by Larry Gibson View Post
Unfortunately your using C.U.P. psi figures and comparing them to the psi obtained by electronic means (piezo transducer or strain gauge). The two are not the same. The "50,000 psi" you refer to is the C.U.P. method using copper cruchers. The MAP (Maximum Average Pressure) for the .308W/7.62 NATO is 62,000 psi as measured by piezo transducers or strain gauges.
No I'm not comparing CUP to PSI, I was trying to point out to others that THEY are confusing CUP and PSI units. I understand completely that the 50,000 PSI value is actually a CUP value and that the actual SAAMI MAP value is 62,000 PSI. Go back to my original post and I would think that it was quite clear that this was my point. By the way, there is a correction that I need to make in regards to my original post about proof loads. I mentioned 74,500 PSI as the proof load pressure but that is incorrect. It is actually a range of 83,000 - 89,000 PSI. And yes, that mistake was caused by reading the SAAMI proof load specs as specified in CUP units rather than PSI. Don't gloat Larry, It wasn't a mistake of misunderstanding, it was a mistake of hurrying and not reading the column heading.

Quote:
Originally Posted by Larry Gibson View Post
In this case it is garbage out only if we mistakenly compare garbage in C.U.P. derived psi's with piezo transducer/strain gauge derived psi's. Use the correct figure of 62,000 psi for the comparison of the data I gave and you'll see that all but 2 loads were under that listed MAP.
I agree Larry, that's the point I'm trying to make, that people need to understand that the actual MAP value is 62,000 PSI and not 50,000 PSI (CUP), as most mistakenly believe. You and I are saying the same thing but we seem to be failing to communicate well with each other.

I would like to add one more question, can you buy me some of that fancy equipment that you use for this stuff? I could use the test receiver and barrel too.

Thanks from High Hat
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