View Full Version : Technical: 7.62mm NATO headspace clarification

February 28, 2003, 13:05
There is a conflicting info on 7.26 NATO minimum headspace
from 2 credible sources:

by gary.jeter February 24, 2001 18:28
1.632 (7.62 NATO minumum headspace)

7.62mm NATO minimum chamber headspace specification is 1.6355

Which number is correct?

Thanks a lot.

February 28, 2003, 13:22
From the FAQs FORUM (http://www.falfiles.com/forums/search.php?s=&action=showresults&searchid=155842&sortby=title&sortorder=ascending) .

I can tell you what the British and Australian and Canadian manuals say.........

February 28, 2003, 14:02
This headspace confusion stems from the use of both .308 win and 7.62 x 51 NATO ammo in the same gun. Nato ammo has thicker brass and can withstand a larger headspace than 308 win ammo. The Nato headspace dimensions are larger than normal because of the extremely adverse abuse the military guns take. So they designed the thicker brass so that it is safe at extremely large headspaces.

When you take all this into consideration.....You must remember that someone might shoot .308 win ammo in your gun. If you set the heeadspace at 1.6355 then it is going to reach no-go teritory for 308 win real quick. So to maintain safety you must make sure that the headspace stays within 308 win safe headspace dimensions. Typically for the FAL we 1.632 as the go-guage and 1.636 as the no-go. Headspace anywhere in this range is safe for Nato or 308 win ammo.;)

1.630" Forster GO
1.630" Winchester .308 minimum
1.6315" NATO 7.62 minimum
1.632" Clymer GO
1.634" Forster NO-GO
1.636" Clymer NO-GO
1.638" Forster FIELD
1.638" Winchester .308 maximum
1.640" Clymer FIELD
1.640" NATO 7.62 maximum

February 28, 2003, 16:09
Originally posted by boyanzhu
Which number is correct?

The second number you give is from this statement: "For example, the US Rifle, M14, 7.62mm NATO minimum chamber headspace specification is 1.6355.""

I don't know enough about the M14 to say if that's correct or not, for that rifle. But notice, it is not a general statement. It's a statement about a particular rifle, the M14 in military service, using NATO ammo.

In other words, <B>it's a rifle spec,</B> as stated, and NOT a general headspace recommendation.

Also notice, the same paragraph goes on to mention, "the .308 Winchester minimum chamber headspace specification of 1.630."

It's important to read things in context. :)

For FALs used by civilians, who may shoot either milsurp or commercial ammo, the SAAMI specs should be followed.

March 02, 2003, 11:55
Well, I can't say which is best. But I can list the spectrum from Go to NO-GO as specified in a certain L1A1 manual.

inch units
1.6325- ACCEPT/GO
1.6380- REJECT/NO-GO
1.6400- CONDEMN

metric units
41.465- ACCEPT/GO
41.605- REJECT/NO-GO
41.656- CONDEMN

AND the C1 manual says:
1.6315" Base AND Field minimum
1.640" Field max
1.6378" when fittin geither NEW barrel or LS

March 02, 2003, 14:07
It's also interesting to note that some sources indicate a "minimum" 7.62x51 headspace (FN = 1.6315 for example) while the source EMDII noted shows 1.6325 as "Accept".

In the Stevens book the Australian manual also shows 1.6325" as "Accept" and goes on to state [Book Two, page 204]:

"To ensure a Rifle is allowed maximum 'life' before the condemning limit (1.640 inches) (41.656mm) is reached, during repair and possible replacement of the shoulder locking, the cartridge headspace should be held as closely to the ACCEPT limit (1.6325 inches (41.465 5mm) as possible."

Pa. Patriot
March 03, 2003, 08:34
Saami 308Win:
GO 1.630
NO GO 1.634
FIELD 1.638

GO 1.6325 (FN & Brit/commonwealth) (Canadian is 1.6315)
NOGO 1.638
FIELD 1.640

GO 1.6355
NOGO 1.638
FIELD 1.6445

That said, a little note about gauges.
Field gauge IS the service check guage.
NO-GO is a armory, rearsenal, re-barrel etc guage. Never intended as a service guage. NO-GO is an armory no-go. Build the rifle with HS *anywhere* between the go and no-go and send it into svc. Most people who roll thier own prefer to get HS as close to min as possible but many obsess unnecessarily over it.

As for the Steven's book quote: "To ensure a Rifle is allowed maximum 'life' before the condemning limit (1.640 inches) (41.656mm) is reached, during repair and possible replacement of the shoulder locking, the cartridge headspace should be held as closely to the ACCEPT limit (1.6325 inches (41.465 5mm) as possible."
Well, Stevens is entitled to his opinion but I bet that armory builds/rebuilds did NOT take care to "minimum HS" the rifles.
They HS'd between GO and NOGO, period. Out the door they went. IIRC there was a former FAL armorer that mentioned this as his personal experience on this board at one time...

{EDIT; typo'ed the #'s}

March 22, 2003, 06:12
Confusion by certain inidividual(s) over M14 minimum headspace of 1.6355" being "NATO 7.62 minimum headspace" is what got this "1.632 is too tight for FN FAL" crap started in the first place.