.308 vs 7.62 X 51


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andym79
March 31, 2012, 08:08 AM
Hi guys I am looking at buying a Ishapore 7.62mm X51 rifle. i have read that using the NATO ammo in a .308 is okay, but not the other way around!

Apparently the average head space in the NATO chamber is .013" longer than in a 308, and the potential for case separation at the head exists!

So I have a couple of questions:

1) Does anyone use 308 commercial in the NATO chamber?

2) Maybe I am barking up the wrong tree here, but could I take a 30-06 cartridge and convert it to a 7.62 NATO, using a 308 FL die and adjusting the die so that the shoulder was forward by .010" and leave the rest to fire forming? Even if the neck had to be .010" shorter would that matter?

Also converting 30-06 would probably make the brass thicker like a 7.62 NATO!

I don' want a case separation!

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Grumulkin
March 31, 2012, 08:26 AM
1. You can use commercial 308 Winchester loads in a 7.62x51 chamber. Head separations will eventually occur after a number of reloadings even with military brass so you need to check the brass for thinning above the web starting after the first or second reload if you wish to be safe. If you run a wire with the tip bent to 90 degrees (I use a large paper clip) you can feel the groove starting on the inside of the case before a head separation occurs.

2. You could make 7.62 NATO brass from 30/06 Springfield brass but in my opinion, it's too much trouble to go to given the great availability of 308 Winchester/7.62 NATO brass.

sniper5
March 31, 2012, 08:32 AM
There's lots of stuff written on this subject, so a search should bring up more than you can possibly digest. From what I've gathered here is the Reader's Digest version:

7.62x51 vs .308 are made with different standards NATO vs SAMMI which FOR THE MOST PART overlap. The external dimensions of the cases are the same, but the internal dimensions of the NATO casing is smaller due to a thicker head to prevent head separation. The chamber specs of the NATO rifle is looser to allow dirty ammo to cycle reliably. The primers of NATO ammo (with the general exception of match ammo) are crimped into place which will only be relevant if you reload, and you can read all about crimped primers and how to deal with them if you do. NATO primers are equivalent to a magnum rifle primer to provide surer ignition under a variety of conditions. The chamber pressure of the NATO rounds runs to the lower end of the spectrum for SAAMI loads although comparing CUP to PSI is apples to oranges and is not a linear comparison.

So what should you do?

If possible buy 7.62 x 51 NATO ammo or it's commercial equivalent. For instance-Winchester white box is a NATO spec ammo and is so marked on the box (it says 7.62 x 51) along with the .308 designation. Or purchase surplus brass like Lake City and reload according to 7.62 x 51 specs with military spec components or their equivalents (for instance CCI 34 primers and IMR 4895 powder). Or use commercial ammo toward the lower end of the power spectrum. You should not have a head separation on a single firing of commercial ammo. If you reload and use commercial brass consider neck sizing to allow the brass to fire form to the chamber and minimize case "working" and load toward the lighter end of the spectrum staying between 55,000 to 60,000 PSI for your chamber pressures.

I don't know if this muddies the water or clears it, but my Ishy gets a steady diet of Lake City brass, CCI 34 primers, IMR 4895, and 150 gr FMJ bullets and I'm using neck sizing dies and marking the serial # of the rifle on the box so the ammo is custom fitted to the chamber. It seems to be happy and smiles when I feed it.

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