I found this in an article:
Shooting .308 Out of 7.62 NATO Chambers
Rather than provide a precise definition of headspace, take it to mean the minimum and maximum recommended dimensions for the chamber that will allow the gun to fire safely. These are the GO and NO-GO values. There is one more value: the FIELD value. This value is the absolute at-the-limits maximum dimension that is acceptable for a gun chamber for minimum safe operation. If this value is exceeded, the gun is not safe to shoot under any circumstance and must be repaired.
We measure these values through the use of headspace gauges. These are steel slugs of varying lengths precision shaped to fit the chamber of a gun . They generally come in sets of two (GO, NO-GO) or three (GO, NO-GO, FIELD). FIELD is optional since not all chamber specifications have a FIELD limit. FIELD limits are generally found in guns derived from military service rifles. The FIELD limit means exactly that: it is the limit that, if exceeded, the gun will not be issued for use in the field by troops.
Now for the technical bit.
For guns chambered in 7.62 NATO, here are the headspace limits (in inches):
And for .308 Winchester chambers:
Put simply, 7.62 NATO chambers are longer. Yes, we are talking about a few thousandths of a inch but it is important. Notice from the limits above that a 7.62 NATO chamber should be able to happily swallow .308 GO and NO-GO gauges. In a .308 rifle, this would indicate the rifle might be unsafe to shoot. For a 7.62 NATO gun, it is still well within its acceptable GO limit.
This is the problem in a nutshell and why the FIELD check is so important. If a 7.62 NATO gun doesn't chamber on a .308 FIELD gauge, it means the gun is probably safe to shoot .308 Winchester ammunition since it means it is within the safe chamber limits for that cartridge, specification-wise.
If a 7.62 NATO gun closes on the .308 FIELD gauge, it is not safe to shoot .308 Winchester. Look at the dimensions. If the 7.62 NATO gun swallows the 1.638" .308 FIELD gauge, it is still within the safe range for 7.62 NATO ammunition with a NO-GO limit of 1.6405"! You would have to use 7.62 NATO NO-GO and FIELD gauges to ensure the gun would be safe to shoot at all if it swallows the .308 FIELD gauge.
If the gun closes on the 7.62 NATO NO-GO gauge, you must use the 7.62 NATO FIELD gauge to ensure the gun is safe at all. If it closes on that gauge, your 7.62 NATO rifle is a wall hanger.
Thousandths of an inch do matter in this game.
Reverse this process and you see why a .308 Winchester chamber can shoot any 7.62 NATO round. Even the .308 FIELD limit is less than the NO-GO limit for 7.62 NATO. As a result, it will safely chamber 7.62 NATO ammunition. Remember, we are checking chamber dimensions, not cartridge dimensions. 7.62 NATO and .308 Winchester cartridge sizes are identical.
We use the headspace gauges to ensure our chamber dimensions are between the GO and NO-GO limits. This is ideal. It means the chamber is well within the NATO or SAAMI .308 specifications. In fact, the ideal chamber length for 7.62 NATO and .308 Winchester is 1.631" to 1.632", which would be very tight in a 7.62 NATO rifle but middle of the road for .308 Winchester.
Ideally, we would use all six gauges in the following sequence to determine where our headspace lies:
.308 GO 1.630"
.308 NO-GO 1.634"
7.62 GO 1.6355"
.308 FIELD 1.638"
7.62 NO-GO 1.6405"
7.62 FIELD 1.6455"
Using the list ordered as above, you can now see why the .308 FIELD check is important for determining whether or not .308 Winchester is safe to use in a 7.62 NATO rifle.
If our gun closes on the .308 NO-GO gauge but doesn't close on the 7.62 GO gauge, we know our headspace is between 1.634" and 1.6354". This would be a nice, tight chamber. It would be safe to shoot both types of ammunition.
If our gun closes on the 7.62 GO gauge but fails to close on the .308 FIELD gauge, we know the headspace is between 1.6355" and 1.637". Likewise, this gun would be safe to shoot both types of ammunition.
However, if our gun closes on the .308 FIELD gauge and doesn't close on either the 7.62 NATO NO-GO or FIELD gauges, the gun can only safely shoot 7.62 NATO specification ammunition. At that point, the chamber dimensions are outside of the safe range for .308 Winchester.
It means the chamber is too large for .308 Winchester and if you shoot it out of such a gun, you are potentially facing a number of unsafe scenarios. On the low end, the thinner .308 brass could suffer head separation on extraction due to brass overexpansion.
Worse would be minor case ruptures with gas leakage and blowback against the bolt face and locking lugs. Moving up the scale you can run into dangerous overpressure issues where the firing parts can be damaged and the very worst would be catastrophic failure of the bolt or chamber. This can result in broken metal going in a variety of directions as shrapnel, hot, high-pressure gas being released or outright destruction of the receiver, all of which occurring directly in front of the shooter's face.
Bear in mind that shooting .308 ammo out of a 7.62 NATO chamber that is beyond the safe limits for it may cause no problems initially. Some guns may never suffer a problem if you do this without knowing any different and a lot of shooters do. But you are potentially gambling with your life if you run out of safe days and combined incremental overstressing of the gun finally causes it to fail.
You could never have a problem or your first shot could be your last. Armed with knowledge, why take the chance?
To answer NJT's question, what you will need to check your rifle will be at a minimum a .308 FIELD gauge. Brownell's carries them. This will determine whether or not it is safe to shoot .308 Winchester out of your rifle. If your rifle closes on this gauge, you can't shoot .308 Winchester safely. At this point, you would be advised to get a 7.62 FIELD gauge just make sure the rifle is safe to shoot 7.62 NATO at all. But that is a "just to be sure" confirmation. Odds are good the rifle is fine on 7.62 NATO headspace. The 7.62 NATO gauges are harder to find and I am looking for a source.
Checking the headspace using the gauges is gun-specific. Many guns require the bolt to be partially disassembled (such as removing the firing pin or extractor). Usually, the check requires the bolt to be closed gently by hand pressure without resistance. Check the armorer's or service manuals for your specific rifle to determine the proper procedure.
I hope this information helps anyone understand the specifics of 7.62 NATO vs. .308 Winchester in 7.62 NATO guns. It can be a confusing subject.
Let me know in comments if you need more information.