One of the most pernicious of these “urban legends” is that there is a significant difference in the pressures between the 7.62x51mm NATO cartridge and the 308 Winchester cartridge. The misinformation indicates that using the commercial offering in a military weapon will visit death and destruction of biblical proportions upon the miscreant who would attempt such a thing.
The Cartridges
The .308 Winchester is a rifle round and is the commercial version of the military 7.62x51mm NATO centerfire cartridge. The .308 Winchester was introduced in 1952, two years prior to the NATO adoption of the 7.62x51mm NATO or T65 round as it was known during testing. Winchester (a subsidiary of Olin Corporation) branded the cartridge and introduced it to the commercial hunting market as the 308 Winchester. Winchester's Model 70 and Model 88 rifles were subsequently chambered for the new cartridge. Since then, the .308 Winchester has become one of the most popular short-action big-game hunting cartridges in the world. It is also commonly used for civilian target shooting, military sniping, and police sharpshooting.
The purpose of the T65 was to achieve the same or similar performance of the then-standard 30-06 cartridge in a package that was more conducive to reliability in fully automatic weapons and infantry weapons under extreme conditions. A weight savings was a by-product of the project, but it was not a
primary consideration
While Winchester intended the T65 (later named 7.62x51mm NATO) and 308 Winchester ammunition to be identical and fully interchangeable, there are some differences. The two primary differences are the specification of chambers size between the two, and the construction or the cartridge case.
Chamber Size
With many military rifles, the chambers can be significantly longer than, say, a Remington 700. Note that the military chamber would fail a NO GO check with a SAAMI gauge, but pass a FIELD check using the proper military gauges.
There is a .013" difference in acceptability, between these two specifications. This is significant in that, for reloading purposes, brass will stretch more in a military chamber upon firing, thereby reducing the life
of the brass and possibly promoting case head separation. But that additional length will allow a round to chamber in an incredibly dirty weapon, which is a requirement for military applications. However, it must be noted that this is the chamber specification and not the ammunition specification. The external dimensions of the two types of ammunition are nearly identical.
Pressure
Despite working together, the two main industry standards organizations SAAMI and C.I.P. have assigned different standards for some cartridges. This leads to officially sanctioned conflicting differences between European and American ammunition and chamber dimensions and maximum allowed chamber pressures. Under SAAMI proof test procedures, for bottlenecked cases the center of the transducer is located .175" behind the shoulder of the case for large diameter (.250") transducers and .150" for small diameter
(.194") transducers. For straight cases the center of the transducer is located one-half of the transducer diameter plus .005" behind the base of the seated bullet. Small transducers are used when the case diameter at the point of measurement is less than .35". Under C.I.P. proof test standards a drilled case is used and the piezo measuring device (transducer) will
be positioned at a distance of 25 mm from the breech face when the length of the cartridge case permits that, including limits. When the length of the cartridge case is to short, pressure measurement will take place at a cartridge specific defined shorter distance from the breech face depending on the dimensions of the case.
The difference in the location of the pressure measurement gives different results than the C.I.P. standard. According to the official C.I.P guidelines the .308 Winchester (referred to as 7.62x51 by CIP) case can handle up to 415 MPa (60,190 psi) piezo pressure. In C.I.P. regulated countries every rifle cartridge combo has to be proofed at 125% of this maximum C.I.P. pressure to certify for sale to consumers.
The .308 Winchester and 7.62x51mm NATO cartridges are not identical and there are minor differences in their inner case dimensions, though SAAMI does not list either cartridge as unsafe in a firearm designed for use with the other. [
http://www.saami.org/Unsafe_Combinations.cfm.
NATO EPVAT testing is one of the three recognized classes of procedures used in the world to control the safety and quality of firearms ammunition. EPVAT Testing is described in unclassified documents by NATO, more precisely by the AC/225 Army Armaments Group (NAAG).
EPVAT is an abbreviation for "Electronic Pressure Velocity and Action Time". This is a comprehensive procedure for testing ammunition using state-of-the-art instruments and computers. The procedure itself is described in NATO document AC/225 (Com. III/SC.1)D/200.
Unlike the C.I.P. procedures aiming only at the user's safety, the NATO procedures for ammunition testing also includes comprehensive functional quality testing in relation with the intended use. That is, not only the soldier's safety is looked at, but also his capacity to incapacitate the enemy. As a result, for every ammunition order by NATO, a complete acceptance approval on both safety and functionality is performed by both NATO and the relevant ammunition manufacturers in a contradictory fashion.
For this, a highly accurate and indisputable protocol has been defined by NATO experts using a system of reference cartridges.
The civilian organizations C.I.P. and SAAMI use less comprehensive test procedures than NATO, but NATO test centers have the advantage that only a few chamberings are in military use. The C.I.P. and SAAMI proof houses must be capable of testing hundreds of different chamberings requiring lots of different test barrels, etc..[7.62 mm. STANAG 2310 and NATO Manual of Proof and Inspection AC/225 (LG/3-SG/1) D/9.]
The US Army continues to use (as of 1995) the M-11 Copper Crusher device for pressure measurements of small arms ammunition. The M-11 was enhanced, when in 1982, it was noted that the results generated at the high end of the test range did not meet NATO standards.
What is interesting to note is that around the time of the engineering change to the M-11 Copper Crusher device, the US Army changed the units of measurement for the device from PSI to Copper Units of Pressure, or CUP. Both SAAMI and CIP used the copper crusher method until the advent of inexpensive, reliable piezoelectric strain gauges, at which point, both organizations converted their methodologies to take advantage of the newer technology. The copper crusher method was the standard for small arms pressure measurements since the late 1800s. A copper pellet just like a small watch battery in placed in the test pressure chamber which is
attached to the cartridge chamber, the test round is fired and the copper pellet is then measured with a micrometer. The micrometer measurement is then converted into a PSI reading by using a chart that converts the length of the pellet into a pressure reading. The charts are constructed using the theoretical modulus of compression for the particular copper alloy used in the pellet, and may or may not have any relation to the actual absolute pressure. BUT, the results of the copper crusher method are always
relative to previous results, which allows for determining what is safe and what is not.
Both SAAMI and the CIP have detailed specifications for the arrangement and dimensions of the copper crusher. Because these two systems are not identical, the two crusher standards cannot always agree.
Further, as explained above, CIP crusher ratings are generally a bit higher than SAAMI's due to differences in definitions. Also, SAAMI is generally more conservative with older military rounds, such as the 8mm Mauser.
With the SAAMI methodology, the piston is positioned over the brass case, and the case will rupture somewhere below 20,000 PSI. The resulting sudden jump in pressure under the piston magnifies problems with piston inertia, and this makes the reading more sensitive to parameters such as burning rate, case strength, and true peak pressure. The CIP methodology requires the piston case be drilled at the sensor location, and the benefit is that crusher and piezoelectric ratios are much more consistent from
cartridge to cartridge, allowing them to reasonably use a conversion formula.
Pressure Confusion
However, neither method addresses the figure “50,000 PSI” that is so often misquoted, especially by “expert” sources such as 6mmbr.com and surplusrifle.com. This figure comes from the US Army in various technical manuals, most notably, TM-D001-27.
The real problem is the confusion between the old and the new methods of pressure testing. The old pressure testing method used for the 7.62 NATO cartridge started out life in the 1950s and is still published today in the US Army Technical Manuals. The figures are based on the copper crusher
method in CUP, but are published as PSI. The new method is the piezoelectric strain gauge transducer method; it is the same technology used
today to show an automobile’s oil pressure. The piezoelectric strain gauge transducer pressure method is a direct pressure reading based on an absolute standard, where the older copper crusher method a conversion based on a relative measure and a conversion chart. And this is why you see the difference in the pressure readings, but the older 52,000 CUP is equal to 62,000 PSI (piezoelectric transducer method).
Today, these two methods are called CUP and PSI and the readings are different, but 52,000 CUP equals 62,000 PSI and both are the same pressure, similar to the way 60 MPH equals 100 KPH.
