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.308 Winchester vs. 7.62x51mm NATO

Discussion in 'Rifle Country' started by Thernlund, May 20, 2008.

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  1. Thernlund

    Thernlund Member

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    Looked through a few pages of search results with no answer. Soooo....

    I know very well that .223 Remington and 5.56x45 NATO aren't completely interchangeable (.223 in a 5.56, but not the other way).

    Are the .308 and 7.62x51 interchangeable? More specifically, is anyone using 7.62x51mm NATO rounds in a consumer .308 hunting rifle (like a Ruger M77/MKII)?

    I have an M77 (and will have an M1A one day :D) and I'm seeing a bunch of deals on NATO rounds. Ok ya think? Anything I should know about this combination?

    Any info is appreciated.


    -T.
     
    Last edited: May 20, 2008
  2. MDW GUNS

    MDW GUNS Member

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    Unlike as you said about the .223/5.56 the .308 Win/ 7.62X51 have the same pressures and everything else is the same.
     
  3. 1911Tuner

    1911Tuner Moderator Emeritus

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    They're fully interchangeable in either direction...but they're not exactly the same. The true NATO-spec round will provide a little more headspace in a chamber that's cut for the commercial counterpart...and the case itself is sized a tiny bit smaller overall. The reason is feed reliability in automatic weapons under harsh conditions.

    Occasionally, you may encounter the odd rifle military in which a commercial round from a given lot won't chamber and go to battery easily.
     
  4. Thernlund

    Thernlund Member

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    Thank you both very much. :)


    -T.
     
  5. General Geoff

    General Geoff Member

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    From what I understood, the external dimensions of .308 and 7.62 NATO cartridges are exactly the same. It's the chamber specifications for the two rounds that *may* be different, because 7.62 NATO has a much looser tolerance for headspace. As such, a tight 7.62 chamber should be identical to a .308 Winchester chamber, and can fire either cartridge without issue. A loose 7.62 chamber, on the other hand, should only be used with 7.62 ammunition, because the 7.62 brass is thicker and safely contains the pressure from firing, even if the headspace of the chamber it's in, is such that the brass isn't fully supported and snug against the chamber wall.

    A 7.62 machine gun will likely have a looser chamber, so that it reliably feeds and extracts 7.62 rounds at a high rate of fire and while very hot. But it's not entirely safe to fire .308 rounds out of said machine guns, because of the .308 rounds' thinner brass.
     
  6. 1911Tuner

    1911Tuner Moderator Emeritus

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    I haven't found that to be exactly the case...no pun intended...across the board.
     
  7. General Geoff

    General Geoff Member

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    I'm still pretty sure that the external dimensional specifications are identical, regardless of how close (or not-so-close) a given ammunition manufacturer gets to those specs...


    edit; nevermind, pulled up these dimension specs.

    7.62 NATO:
    Bullet diameter 7.82 mm (0.308 in)
    Neck diameter 8.58 mm (0.338 in)
    Shoulder diameter 11.35 mm (0.447 in)
    Base diameter 11.84 mm (0.466 in)
    Rim diameter 11.94 mm (0.470 in)
    Rim thickness 1.27 mm (0.050 in)
    Case length 51.05 mm (2.010 in)
    Overall length 69.85 mm (2.750 in)

    .308 Winchester:
    Bullet diameter 0.308 in (7.8 mm)
    Neck diameter 0.343 in (8.7 mm)
    Shoulder diameter 0.454 in (11.5 mm)
    Base diameter 0.470 in (11.9 mm)
    Rim diameter 0.473 in (12.0 mm)
    Rim thickness 0.050 in (1.3 mm)
    Case length 2.015 in (51.2 mm)
    Overall length 2.800 in (71.1 mm)
     
  8. Thernlund

    Thernlund Member

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    If you got those from Wikipedia, be careful about taking that to seriously. There's alot of bickering over there about that stuff. I used to be a part of it and finally gave up.

    (I wrote/adapted the ammo sidebar and the conversion code for the inches to mm conversion.)


    -T.
     
  9. General Geoff

    General Geoff Member

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    from this article:


    There's more in the article, it's an interesting read.


    edit; definitely read the article, because the charts don't work well in pure text form. :)
     
  10. Thernlund

    Thernlund Member

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    Thanks a bunch! :)


    -T.


    EDIT: Eeek. The article starts out...
    I guess that's me. :(
     
  11. General Geoff

    General Geoff Member

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    Here's another article, from Fulton Armory:



    For what it's worth, my Springfield M1A (manufactured in December of '06) has a headspace of 1.632, so I know I'm safe (as long as I stick to 175gr or lighter bullets due to the gas system), whether I load up .308 or 7.62 in my rifle.
     
  12. USSR

    USSR Member

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    From a previous post of mine, in regards to the perceived differences in pressure:

    The following is from another poster who added additional info:

    Don
     
  13. Candiru

    Candiru Member

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    USSR, do you know what the original government document describing the 7.62x51mm NATO spec is? I've been trying to get my hands on a copy in the interests of being able to point to documented proof showing that the perceived pressure difference stems from different measuring methods. I'm pretty sure this is actually the case, but I'm just going off people's say-so; I'd like to spread the word but don't want to say something unless I know it for a fact.
     
  14. 1911Tuner

    1911Tuner Moderator Emeritus

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    Generally, pressure readings in CUP...Copper Units of Pressure...wil turn up lower numbers than PSI...or Pounds per Square Inch.

    i.e. 50,000 CUP usually equates to around 52,500 PSI.

    I use the terms "Generally" and "Usually" because CUP readings can vary with the case construction. Wall thickness...Web thickness...Malleabilily of the case material, etc.

    It's entirely feasible that a given PSI reading wll produce different CUP readings with a difference in the brass. Thin, soft brass will give a different CUP reading than thick, hard brass,even though the actual pressure in PSI is the same...but assuming that all else is equal...CUP numbers will be a bit lower than PSI.
     
  15. Jim Watson

    Jim Watson Member

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    The difference in CUP and piezeo psi is usually more than 5% for high intensity (How many here remember THAT term?) or magnum bottleneck rifle cartridges. I have an IMR pamphlet that shows both. The .270 tops out at 52,000 CUP but the maximum load in the psi section reads 62,000. Be more interesting if they were for the same load, but this is a sign that the two methods diverge a lot.

    Note the 50,000 vs 62,000 numbers in the 7.62 vs .308 scare piece; same deal, way more than 5% difference in the methods.


    If you think that is tough, try translating British "ton" proof numbers into US practice. Gough Thomas did a piece on that and concluded that like to like, a British proof ton amounted to about 2800 US psi (probably really CUP) at shotgun levels.
     
  16. 1911Tuner

    1911Tuner Moderator Emeritus

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    Shotguns! Ah, yes...and now we have another method of measuring pressure. Namely...LUP. Lead Units of Pressure. Here, a lead crusher is used instead of copper.

    Clear as mud...ain't it?

    *sigh*

    Raises right hand...
     
  17. Jim Watson

    Jim Watson Member

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    LUP is normally for shotshells which run at much lower pressure. A copper crusher would not consistently deform.
     
  18. USSR

    USSR Member

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    The original government document states "50,000psi", but they don't tell you that it was obtained using the copper crusher method. So, you cannot compare it to SAAMI's psi pressure spec's for commercial cartridges which in recent years were obtained using the piezo method, which is what everybody DOES before pronouncing the .308 Winchester producing MUCH higher pressure than the 7.62x51 round. Since we have both CUP and psi pressures for the .30-06 (50K CUP and 60K psi), it is not a stretch to say that the 7.62x51 at 50K CUP would produce nearly 60K psi via the piezo method. And, the difference between a 60K psi 7.62x51 round and a 62K psi .308 round is negligible. Hope that helps.

    Don
     
  19. Schleprok62

    Schleprok62 Member

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    Clear as mud... but, it's straining through the brain filters slowly....

    *raises right hand*

    I need to use the hall pass...



    Actually, this is good educational stuff, although, I have to do some other searches and readings to finally understand some of it... but it's great infromation.
     
  20. SMLE

    SMLE Member

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    I keep posting this and nobody reads it...

    http://www.smellysmleshooters.net/ammopressure.htm
     
  21. JCUMM2

    JCUMM2 Member

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    One important issue pertaining comercial 308 ammo versus 7.62 Nato ammo is the powder used and the rifle. Specifically the burn rate needs to properly match the intended burn rate in semiauto rifles. Some commercial 308 ammo does not use a powder with the proper burn rate for the M1 Garand or the M14/M1A. I'm very cautious about using commercial ammo in either of these unless I know for a fact they are OK. For my M1 asd M1a I use either military ammo, very carefully handloaded ammo (loaded be me and every piece gauged), or rarely Federal Gold Metal Match. In particular I would be cautious about using 308 hunting ammo in either of these guns as they can run higher pressures and too slow powders.
     
  22. .45&TKD

    .45&TKD Member

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    My understanding is that would be OK. But the reverse is not so.

    For example, don't shoot standard pressure commercial .308 in an Ishapore
    2a Enfield chambered for 7.62x51mm NATO .
     
  23. SMLE

    SMLE Member

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    You CAN shoot commercial, FACTORY loaded .308 in an Ishapore 2A1 rifle. Commercial .308 is LOWER pressure than military 7.62X51 NATO.
     
  24. .45&TKD

    .45&TKD Member

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    Not according to Surplusrifle.com

    http://www.surplusrifle.com/shooting2006/308vs762nato/index.asp
     
  25. sernv99

    sernv99 Member

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    so I can't shoot 7.62x51 in my PTR-91? I have heard many people do it and it is advertised as being chambered for .308 caliber.
     
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