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Differences: .308 and 7.62x51 NATO

Discussion in 'Rifle Country' started by MarineOne, Nov 13, 2009.

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  1. MarineOne

    MarineOne Member

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    I just picked up one of the many gun magazines and it's got a new rifle from LMT that fires the .308 Win instead of the 7.62x51 NATO. What are the differences in these two rifle rounds?


    Thanks.
     
  2. TexasRifleman

    TexasRifleman Moderator Emeritus

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    There is no difference, there is a difference :)

    The differences are slight and in most firearms it won't matter. In semi auto's you need to watch headspace.

    I'll just repost what Walt Kulek over at Fulton Armory says about it:

    Clint McKee at Fulton says this:

    You may read things about pressure differences. SAAMI uses one type of pressure measurement and the Europeans used a different method.

    There is an great ongoing debate about whether the 2 pressures are 20,000 psi apart or if they are actually the same. The argument centers around what appears to be a typo in an old army field manual.

    It's a very complicated and convoluted argument but I've come to the conclusion that for me personally I do not believe there is enough of a chamber pressure difference to worry about it. The whole debate and documentation is over at FAL Files if you feel the need to dig for it.

    It's your gun and your life so read all the advice and choose carefully what you believe.
     
  3. Mark whiz

    Mark whiz Member

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    Ditto to TexasRifleman.

    Since you are chambered for .308 - you can shoot either without having to worry.
     
  4. Balrog

    Balrog Member

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    7.62 brass tends to be a little thicker at the base, and has a little less volume capacity than commercial 308. That is important if you are a reloader.
     
  5. Candiru

    Candiru Member

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    Regarding the pressure difference, the following is what I've come to believe. The Army manuals well back into the 50s listed the pressure for rounds as PSI, although it wasn't until the 70s that they started using the electrical pressure sensors that actually measure pounds per square inch. Up to that point, they used the copper crusher method, which actually measures CUP, or Copper Units of Pressure. CUP doesn't translate into PSI using any mathematical equation, but it's known that 50,000 CUP is roughly equal to about 60,000 PSI. These numbers match up with the apparent discrepency between 7.62 NATO and .308 very nicely.

    The most convincing thing, though, is just common sense. The US Army Field Manual for the M14 lists the muzzle velocity as 2,800 FPS. (link to FM 23-8) and .308 ammo manufacturers publish their velocity numbers; for instance, Remington claims its .308 150-grain FMJ cartridge (link) gets 2,820 FPS. Would the supposed 12,000 PSI difference between 7.62 and .308, a nearly 20% increase, produce only 20 extra FPS velocity?
     
  6. scythefwd

    scythefwd Member

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    candiru, if the military is using a faster powder it very well could spike the pressure earlier and higher but have a lower velocity than if it used slower burning powders. Slower burning powders have a lower peak pressure, but a longer interval of time at that pressure (or close to it) where fast burning powders reach peak quicker and drop back off faster which results in slower bullets. For fast burning powders, it is more of a pressure spike instead of a pressure curve (to an extent.
     
  7. MarineOne

    MarineOne Member

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    I was thinking about picking up this new rifle from LMT, but I'll stick with an M1A/M14 from Springfield. My wife said she'd get me something for Christmas, and since I won't be home until late summer I'd rather her get a deal now than for me to hunt for it while I'm home and limited on time.

    I appreciate the input.



    Kris
     
  8. TexasRifleman

    TexasRifleman Moderator Emeritus

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    Well this is why it's a can of worms debate, there are lots of arguments on each side.

    So the argument the other way goes that since a very common load for 7.62 uses BL(C) powder then we should be able to duplicate pretty close the load of the military rounds.

    BL(C) in Hodgdon's loading data shows 50,000 psi with a 150gr FMJ, at roughly the same velocity as the NATO load.

    So, if BLC2 was developed for NATO loads, and NATO loads use the same bullet as a .308 load, and the velocities are the same, how can there be a 10-20,000 psi chamber difference?

    That's why I personally have decided to ignore that whole pressure thing, but it's certainly a personal choice.
     
  9. scythefwd

    scythefwd Member

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    Well Tex, thats only if the military round uses BLC2. I can't say either way. In general, the 7.62x51 is considered a higher pressure round, but negligibly so. I'd say it's your gun but I'd use either if I were you.
     
  10. TexasRifleman

    TexasRifleman Moderator Emeritus

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    Well, TM 43 says that 7.62 M80 Ball ammo uses WC846. (Page 11-17) 46 grains of it.
    http://www.ar15.com/content/manuals/TM43-0001-27.pdf

    Pretty much everything I can find says that commercially that powder is sold as BLC-2 or H335. H335 is salvaged/surplus WC846 and BLC2 is newly manufactured at least according to what I can find.

    So who knows. It's close enough that for me personally I follow the advice of the guys at Fulton and as long as the headspace is OK I don't worry about using either one. But if someone wasn't absolutely certain themselves I wouldn't fault them at all for not interchanging the things.
     
    Last edited: Nov 14, 2009
  11. jonnyc

    jonnyc Member

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    They are the same round and factory ammo of either flavor is perfectly interchangeable. A "match grade" commercial .308 chamber might not like the looser manufacturing tolerances of military 7.62 ammo, but that's a rifle issue...not an ammo issue. The only difference between .308 and military 7.62 is the internal construction of the case. Military brass has a thicker web to withstand MG extraction, thus there is a slightly reduced internal volume. One might have an issue if they tried to put a hot .308 load into a military case.
     
  12. Candiru

    Candiru Member

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    Yeah, that follows. If the military loads produced slower muzzle velocities, that would likely be the cause. However, the military loads have everything working against them in terms of velocity (reduced case size and older, faster powder), so you'd expect them to be clearly slower if commercial loads had an extra 12,000 PSI to play with.

    It's possible to argue that commercial loads in the 150-grain FMJ range are underloaded to make them safe in military arms, but if that were the case you'd expect to see hunting loads with heavier bullet weights at the same speed as the 150-grainers.


    That's CUP they're using, and 50,000 CUP is about 60,000 PSI.
     
  13. TexasRifleman

    TexasRifleman Moderator Emeritus

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    Yeah, keeping them straight is always the problem lol.
     
  14. Hammerhead6814

    Hammerhead6814 Member

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    No difference.
     
  15. desidog

    desidog Member

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    When i was researching the 7.62 CEMTE round for my FR-7 rifle, I got the impression that there IS a difference in CUP between 7.62NATO and .308Win; but that in modern rifles the difference is moot, given that guns are under-rated/over-engineered for a safety margin. Either way, it shouldn't blow up in your face!

    Now, the 7.62 CEMTE is another story...it has about 20,000PSI less than the .308, so a mistake there could literally blow up in your face.
     
  16. Mags

    Mags Member

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    The same difference as 223/5.56 it's all in your head. If the brass was so much thicker reloaders would have to load them differently than their commercial counterpart. I don't load the military brass any differently and have never experienced any pressure signs.
     
  17. mljdeckard

    mljdeckard Member

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    When I would shoot .308 in my M1A, I would get failures to extract when it started to get hot. No such problems with 7.62, which is what it was chambered for. For ARs, I only use 5.56 or Wylde chambers and I've never had a problem. They DO load them differently than their commercial counterpart.

    In bolt rifles I have never had a problem. The marine scout-snipers I was working with dais that they know there is a difference, but it doesn't cause a malfunction in their M-40s. As long as they are consistent and they don't switch back and forth, they are ok for accuracy out to 1000 meters. Their ammo was marked 7.62 NATO match.
     
  18. wyohome

    wyohome Member

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    "The same difference as 223/5.56 it's all in your head." Mags

    "Reduce charges developed in commercial cases at least 3 percent when loading military brass." Speer Reloading Manual #14.
    I tend to lean toward the latter..
     
  19. jonnyc

    jonnyc Member

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    "They DO load them differently than their commercial counterpart."
    This is not quite accurate. There are so many different loadings of both .308 and 7.62x51 ammo that you cannot say about any given loading that one is always hotter than another. Then you can throw manufacturing differences into the mix.
     
  20. Mags

    Mags Member

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    It's BS, size only matters in chamber size.
     
  21. Balrog

    Balrog Member

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    You just havent had pressure signs with your load. I have loaded 223 for many many years (you may have also, so I don't mean to sound like I am talking down). If you load military brass to max loads, you will probably see pressure signs in some of your spent cases. I certainly have seen pressure signs in military loads loaded to max.
     
  22. wyohome

    wyohome Member

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    "It's BS, size only matters in chamber size."

    I am guessing that you are joking, but I am afraid you are serious.
     
  23. Mags

    Mags Member

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    Yes I am serious. Yes case volume does affect pressure. No the difference between 223/5.56 and 7.62/308 is not noticeable. I speak from experience, I use the start load listed in a given manual for all new brass, primers, or bullets when working up a load and have never experienced pressure signs regardless of military/commercial cases. I have even shot 5.56 in a Colt chambered in 223:eek: and did not have any negative experiences and I mean alot of 5.56 in the ol 223.
     
  24. wyohome

    wyohome Member

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    "Yes I am serious"

    Well, then I stand corrected. In will notify Speer, Hornady, Lee, Barnes, and the others about their erroneous testing and information on Monday.
     
  25. Mags

    Mags Member

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    Thanks, please do and I am sure their tests probably come up the same as mine, however I am sure they have encountered variables in their volume of testing and wanted to CYA so they post more conservative data.
     
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