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copyright 2001 - Stephen Redgwell
Do you have a military surplus rifle chambered in 7.62 x 51mm? If so, you probably bought it to get a "308 Winchester"
at a really low price. Well, you messed up. It's like looking at a rabbit and a hare. They're close, but each is distinctive,
and you have to understand what you're looking at.
Early in my military career, I worked on the FNC1 rifle (7.62 NATO). For someone that had only handled civilian shotguns
and rifles, this was going to be a real treat! It was only natural that curiosity would generate questions and comparisons
about what I thought was the same cartridge - 308 Winchester and the 7.62x51mm NATO.
 One of my personal rifles was a Midland 2100 chambered for 308 Winchester. It was made in England by a small company - Midland
Rifle Co - but owned by Parker Hale. Since there was a lot of military 7.62x51mm available, I shot them from my 2100 from
time to time. There was never a problem chambering or firing the military stuff. It was a pleasant diversion over the long
(read boring) periods spent at the range for base small arms qualifications.
Around this time, I discovered that shooting reloaded cases fired from an FN was virtually impossible. They didn't want
to chamber. My frustration led me to ask an older armourer what was wrong. In a nutshell, he told me that they probably
didn't fit because they stretched. I was using a Lee Loader in 308 Winchester and didn't know that they only resized the
neck, leaving the rest of the case untouched. The shoulders were blown forward on initial firing, so the case was simply
too long to fit my rifle's chamber! Hmmm...
I tried the gauges from work in my own rifle and was surprised to discover that none of them would fit! The corporal
brought a set in from home and explained the difference. Here's what I discovered.
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308 Winchester (SAAMI) Gauges
GO - 1.6300"
NOGO - 1.6340"
FIELD - 1.6380"
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Shop (Military) Gauges
GO - 1.6350"
NOGO - 1.6405"
FIELD - 1.6455"
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The gauge sets were different too! What gives? He told me that the dimensions of the two new and unfired cartridges
were basically the same. The difference lay in two areas - chamber size and cartridge makeup.
CHAMBER SIZE
Look at the picture below. The top chamber represents a military rifle, the bottom one a commercial sporter. With many
military rifles, their chambers can be significantly longer than, say, a Remington 700. Note that the military chamber would
fail a NO GO check, but pass a FIELD check using the proper shop (military) gauges.
There is a .013" difference in chamber length however, between these two "Safe & Serviceable" rifles!
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| The dotted line represents the point on the shoulder where the cartridge would headspace |
There will be distinct differences between chambers of military rifles based on:
Type - Ishapore, Garand, FN etc.
Amount of Use
Condition at Time of Sale
Just by looking at the dimensions, you can see that using SAAMI gauges is bound to reveal "problems".
CARTRIDGE MAKEUP
I said at the beginning that these two cartridges were basically the same, dimensionally. While they may share similar
external measurements, there is one major difference that you can't see - case wall thickness.
Military brass is thicker. It needs to be. It was
made to function and stretch in a wide variety of firearm chambers. Don't forget that the condition of spent brass is of
no importance to the service. Like 99% of military brass, it won't be reused.
Where does that leave you? Follow
this rule and you won't have any problems.
DON'T USE YOUR RIFLE IF IT FAILS THE SAAMI FIELD CHECK!!
Why not? Right away you know that the chamber is measuring more than 1.6380" (SAAMI FIELD) - possibly more. Commercial
308 ammunition may stretch by 0.008" when fired! Possibly more. It may even rupture! You must get the headspace corrected
to bring it in line with SAAMI specs. Hey, it's for your own safety!
Based on this rule, find your situation
below.
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I DON'T RELOAD AND USE ONLY MILSURP AMMUNITION.
You shouldn't have any problems.
I DON'T RELOAD BUT WANT TO USE COMMERCIAL AMMUNITION.
There is a potential problem with brass integrity.
Commercial cases are NOT as thick as their military cousins. There is less brass, so if you use them in an original, unmeasured
military chamber, there may not be sufficient material to stretch and fill the chamber without rupturing. That's bad!!
I RELOAD.
You know that commercial brass is thinner. An easy way to see this is to weigh a fired case
of each type. You'll see that the commercial case is lighter. There may not be enough brass left - after lengthening - for
any decent case life. You will only aggravate the situation by full length resizing (it weakens the case by overworking the
brass), loading at or near maximum or both.
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If you own a milsurp bolt gun like an Ishapore 2A or 2A1,
consider neck sizing the brass for better case life, fit and accuracy.
SOME FINAL THOUGHTS
Military
ammunition is loaded to maximum average pressure 50,000 CUP (approximately 58,000 PSI using the modern piezo transducer method
of measurement). This standard is used to ensure better consistency round to round. It is proofed at 67,000PSI. The ammunition
can then be used in a wide variety of firearms with no ill effects.
Look for the NATO design mark.
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| NATO mark found on the outside of ammunition containers and the bottom of each case rim. |
Commercial ammunition has a SAAMI/ANSI maximum pressure of 62,000PSI. While not every manufacturer may load it to this level,
this is the industry established maximum. This is also measured using the piezo transducer method. The proof cartridge pressure
is 83,000 to 89,000 PSI. Note the differences between the military test and operational standard vs the commercial one.
The worst case scenario for shooters of 7.62 NATO rifles is as follows: Using a commercially made, maximum pressure cartridge
with long headspace, fired from a weak action (ex. converted 93/95 Mausers).
The weaker action is a possible final piece of the 308 vs 7.62 NATO puzzle.
Perhaps no single element will cause your rifle to fail. Maybe you've used your rifle for a while with no ill effects.
Usually a rifle will take some abuse before failing. Like a lot of things in life, Murphy's Law - If something can go wrong,
it will - could very well catch up with you.
Personally, I don't like the odds.
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