What KurtC said is true, there is no difference between .308 Winchester and 7.62 NATO with regards to maximum chamber pressure. I have a Dept. of the Army manual dated April 1994 (TM43-0001-27) that states that the chamber pressure of 7.62 NATO ammo is 50,000 PSI, this is wrong, 7.62 NATO was originally measured using CUP (copper units pressure), a copper crusher system. It was wrongly transposed to PSI. If you google search SAAMI and look in the rifle cartridges section you will find that .308 Win and 7.62 NATO have the same pressure ratings. The issue with regards to M14 type rifles is that the gas system was designed to operate at, if I remember correctly, 12,500 +- 2,000 psi port pressure. This would mean that when handloading ammo you only have a certain area of applicable powders that will work and maintain that port pressure so as not to damage your gas system. And please, no offence to anyone but when will these myths finally die, it's almost as bad as the 5.56/.223 debate.
John
I lament the re-classification of older versions of TM 43-0001-27 as Distribution A, and the book in general as a "technical manual". This is because as a technical manual, people erroneously use it as a source of technical information. It is intended as a source of logistical information, DoDACs and NSNs. It is
NOT a good source of information regarding technical aspects of ammunition, pressures, velocities, sometimes even bullet weights, but especially charge weights.
To quote the stated purpose of the manual:
1-1. PURPOSE - This manual is a reference handbook published as an aid in planning, training, familiarization and identification of small arms ammunition, ranging from .22 Caliber to 30 millimeter and shotgun ammunition from .410 to 10 gage. This is the Federal Supply Class 1305. Operating instructions are are contained in the appropriate weapons manuals. (my emphasis)
The usefulness of TM 43-0001-27 in really limited to logistics planning.
If you want technical information of the characteristics of various ammunition, go to the actual military specifications, which unfortunately are not listed. (However, all but the latest are availble on line.)
SAAMI vs Military
Comparison between SAAMI and military pressure limits is near impossible due to the differences between methods used. For the longest time, the military has measured chamber pressure at case mouth, whereas SAAMI listed the limits at mid-case, there is no fixed correlation between these two. However, the military does use mid-case pressure values on occasion for research and development testing, so we do have historical samples of mid-case pressures for various lots of ammunition. None of these have ever exceeded SAAMI limits, so it is a safe assumption that military is "safe" in all firearms that will actually chamber it.
There are other differences between the the two requirement standards, the military listing the average chamber pressure and limiting the extreme ends of the distribution curve by limits on the standard deviation, SAAMI limits the maximum average chamber pressure and limit the distribution by the Maximum Probable Lot Mean (MPLM) and Maximum Probable Sample Mean (MPSM) which are defined in SAAMI Z299.4.
And last, there is the actual chamber of the pressure test barrel. The SAAMI requirement for the pressure test barrel chamber is the standard chamber for .308 Winchester made to the minimum size. The military pressure test barrel chamber is unique to the velocity and pressure test barrel. (And BTW, that chamber has no leade or freebore, the forcing cone starts at the termination of the case neck.)
Port Pressure
The specified port pressure for 7.62mm ammunition is taken at 15 inches from the breech face. This is not the location of the port in the M14, M60, or M240. So please do not make the error of thinking that a port pressure in excess of 14,500 psi* is damaging to your M1A/M14.
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* This was the highest listed port pressure limit, and should be noted is a C.U.P. measurement. The current specified port pressure limit for M80 Ball ammunition is 9,150 to 12,650 psi.
