7.62x51 (NATO) and .308 Winchester

This is a discussion on 7.62x51 (NATO) and .308 Winchester within the General Firearm Discussion forums, part of the Related Topics category; So, I have a question. First, a little background. My buddy bought a AR-10 and a bunch of Monarch 7.62x51mm (not NATO) and while it ...

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Thread: 7.62x51 (NATO) and .308 Winchester

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    7.62x51 (NATO) and .308 Winchester

    So, I have a question.

    First, a little background.
    My buddy bought a AR-10 and a bunch of Monarch 7.62x51mm (not NATO) and while it fed ok though his AR10 it would fail to feed every 3-6 rounds. A jiggle of the magazine and a pull on the bolt made the next 5 or so rounds go well. He bought bought a new mag and it did the same thing, bought some .308 win and everything was fine.

    I told him I would buy (for next to nothing) the rest of his Monarch ammo because I have a Savage arms .308 lever action carbine and I figured a lever action would take the ammo.

    I shot 1 round through my rifle and it expanded to the point that I could not remove it from the chamber. It was not stuck very hard...I lowered a cleaning rod down the barrel and tapped it out in a second or two. (Yes, I first confirmed the bullet went downrange first. )

    I realize there are slight differences in the requirements for headspace 7.62x51mm and NATO and SAAMI .308 but I have always been told they are pretty much interchangeable.
    My question is this; Is the headspace on my Savage the SAAMI spec and therefore the 7.62x51mm's headspace was small enough to be chambered but when expanded, it wedged itself in? Also, I am hesitant to shoot another round through. In fact, I am thinking of trying to sell the ammo (Yeah, good luck, I know right...) and just stick with the .308 Win. I'd like to hear from some people that know a bit more about ammo measurements than I do...

    Thanks guys!
    -Ian

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    A couple of things. First, NATO-spec 7.62 x 51 ammo is loaded to a lower max pressure than .308 Win - 50,000 psi vs 62,000. So strictly from a safety standpoint, it's OK to shoot 7.62x51 in a .308, but the reverse is not universally true - depends on the gun's specs.

    Further, the chamber dimensions are not identical, with the 7.62x51 having a slightly longer chamber. Thus with the potentially higher pressure of the .308, the case may stretch more than a safe amount to fit the longer chamber, with a possibility of rupturing the case. The likelihood of that happening all depends on the specific chamber and specific ammo.

    Lastly, brass manufactured to military spec has a thicker case thickness, so the internal case capacity is a bit smaller than that of the .308. Thicker brass also means stronger and less easily deformed, which explains the difficulty in extracting the spent case from your gun.

    So, I would abandon the use of 7.62x51 in your Savage (a 99, I presume?) and stick with .308. As for your friend, remind him that AR-platform guns like to run wet with lube, and that alone may clear up his cycling problem. He may also have a weak buffer spring which fails to seat the less elastic mil-spec ammo.
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    Smitty
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    The SAMMI/CIP maximum pressure for the .308 Win cartridge is 62,000 psi, while the 7.6251 max is 50,000 psi. Also, the head space is slightly different. The .308 Win “Go Gauge” is 1.630″ vs. 1.635″ for the 7.6251. The .308′s “No-Go” dimension is 1.634″ vs. 1.6405″ for a 7.6251 “No Go” gauge.

    Most likely it's the crappy quality of that steel cased Russian ammunition.
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    I had a problem with my Eagle Arms AR10 failing to feed Indian Surplus 7.62x51,I was shooting brass cased ammo and it seemed as if the chamber was really tight,so I scrubbed it really good with a bore brush and made sure the BCG was lubed good,haven't had a problem since
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    Thanks guys. I bought some .308 and it fed perfectly. I'll try to get rid of this crap. I appreciate the information and advice

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    Actually, NATO/Military uses a different process to measure pressure, the 7.62 NATO is not loaded to a lesser pressure than .308 Winchester.

    The rated Maximum of a 7.62 NATO Proof round is 67,500 PSI +- 2,500 PSI average.

    The prohibition about shooting .308 in a 7.62 N chamber is that you are creating a case of artificial headspace.

    I have seen extraction problem with 7.62N and hotter .308 Winchester (European loading) when used in the Savage 99 lever actions, in fact Musgrave brand use to print on their boxes "Not for use in Semi Auto or Lever Actions" because the cases were loaded to European standard (CIPP) which tends to be a little hotter than SAAMI spec and would not extract, because there was enough flex in a lever rifles lock up that the case swelled and jammed the bolt and once the bolt was opened the extractor, in this case tore the rim off and we resorted to a cleaning rod.

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