Article: 7.62x51mm NATO or 308 Winchester? What's the Difference?
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Umpteen millions and more rounds of surplus NATO ammo has been fired in commercial .308 winchester chambers.
Tens of thousands of shooters across north america regularly fire commercial .308 winchester ammo in their 7.62x51 chambered milsurps.
Given the sheer numbers in play, there doesn't seem to be a lot of statistical evidence of catastrophic failures as predicted when shooting 7.62 Nato in a .308 winchester, or vice versa. Go figure.
Tens of thousands of shooters across north america regularly fire commercial .308 winchester ammo in their 7.62x51 chambered milsurps.
Given the sheer numbers in play, there doesn't seem to be a lot of statistical evidence of catastrophic failures as predicted when shooting 7.62 Nato in a .308 winchester, or vice versa. Go figure.
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None. What matters is your chamber. There are so many different manufacturers. some make good stuff some make crap. Stay away from berdan primed 7.62 casings from eastern europe they tend to bust depriming pins if they get mixed in.
Good article. But I disagree with one point. He says the commercial ammo is loaded to higher pressures. Not so. Both are loaded to about the same pressure, although the NATO standard was first defined in CUP, not psi, and this has lead to the 50,000 vs 60,000 confusion.
Most military and commercial is loaded to around 55,000 to 58,000 psi. I have seen both flavours at a bit over 60,000, but that was unusual. I had one lot of IVI re-tested because it was causing pressure problems in target rifles (tight chambers and undersize barrels). It was over 60,000 psi but just under the limit and was kept in service.
One way to see that the ammo is similar pressure is to shoot some military ball 147 gr and commercial 150 gr and note that they are about the same velocity. If there was a 10,000 psi difference, there would be a noticeable difference in pressure.
Here is pressure testing data conducted by the Explosives Branch at Bells Corners, Ontario. They certify ammo before it can be sold. The test data was in French. Moyanne means Average.
On one page you can see Lake City Match (NATO) is quite a bit hotter than Norma match (commecial). The Lake City pressures are typical of 7.62/308 pressures. The Norma pressures are a bit lower than usual.
On the next page you can see the SAAMI standardard reference ammo (Winchester commecial) tested against IVI 7.62. This test involves both ammos shot in two different barrels. You can see that the IVI is higher pressure than the commercial and also that the DCRA barrel (a tight Obermyer match barrel with an Obermyer chamber) raises pressures.
These two pages would suggest that NATO is hotter than 308Win. Not so. I have tested hundreds of lots of both and found that, on average, they are the same. Most lots of either flavour run in the 55,000 to 58,000 psi range.
There are two pressure measuring protocols in North America. Up until the late 60's, the copper or lead crusher system was in use. Because it was the only system used, some tables referred to the results as "psi" instead of "CUP" (Copper Units of Pressure). This did not matter since we all knew what was meant.
When the pyzeoelectric transducer came into use, we all became a lot more careful to report results as either CUP or PSI. It would be like not making it clear if car speed was in mph or kph. It makes a difference!
The old military litterature listed CUP pressures as psi, and that litterature is still used. Those in the business know they mean CUP, but because the 7.62 is shown as 50,000 psi, it is often taken at face value. Hence the misconception that NATO is milder ammo.
I am often asked how to convert CUP units to PSI. It cannot really be done. Not only is the measureing equipment different, but the place the pressure is measured varies from caliber to caliber. In a given caliber, one sytem might measure pressure at the neck whereas the other measures pressure in the middle of the case . In some calibers, the limits are higher or lower than the other system, and for a few the limits happen to be the same numbers. You can see the pressure limits under both systems at:
http://kwk.us/pressures.html
Note that they show the 308 Win limits as 62,000 psi and 52,000 CUP.
Because the location of the pressure point varies from cartridge to cartridge one cannot jump to conclusions that,say, a 308 is loaded hotter than a 30-06 or a 243. Each is measured in its own way, which may make pressure readings different, even though the actual pressure readings are the same.
Here is a page from another book where the same ammo was tested by both systems. As you can see, the CUP numbers are smaller, even though the pressures are the same.
I do agree that the NATO chamber can be deeper than a 308. This can lead to head separation if the case is full length sized all the way. Best procedure is to neck size only or FL size only enough to permit chambering.
Most military and commercial is loaded to around 55,000 to 58,000 psi. I have seen both flavours at a bit over 60,000, but that was unusual. I had one lot of IVI re-tested because it was causing pressure problems in target rifles (tight chambers and undersize barrels). It was over 60,000 psi but just under the limit and was kept in service.
One way to see that the ammo is similar pressure is to shoot some military ball 147 gr and commercial 150 gr and note that they are about the same velocity. If there was a 10,000 psi difference, there would be a noticeable difference in pressure.
Here is pressure testing data conducted by the Explosives Branch at Bells Corners, Ontario. They certify ammo before it can be sold. The test data was in French. Moyanne means Average.
On one page you can see Lake City Match (NATO) is quite a bit hotter than Norma match (commecial). The Lake City pressures are typical of 7.62/308 pressures. The Norma pressures are a bit lower than usual.
On the next page you can see the SAAMI standardard reference ammo (Winchester commecial) tested against IVI 7.62. This test involves both ammos shot in two different barrels. You can see that the IVI is higher pressure than the commercial and also that the DCRA barrel (a tight Obermyer match barrel with an Obermyer chamber) raises pressures.
These two pages would suggest that NATO is hotter than 308Win. Not so. I have tested hundreds of lots of both and found that, on average, they are the same. Most lots of either flavour run in the 55,000 to 58,000 psi range.
There are two pressure measuring protocols in North America. Up until the late 60's, the copper or lead crusher system was in use. Because it was the only system used, some tables referred to the results as "psi" instead of "CUP" (Copper Units of Pressure). This did not matter since we all knew what was meant.
When the pyzeoelectric transducer came into use, we all became a lot more careful to report results as either CUP or PSI. It would be like not making it clear if car speed was in mph or kph. It makes a difference!
The old military litterature listed CUP pressures as psi, and that litterature is still used. Those in the business know they mean CUP, but because the 7.62 is shown as 50,000 psi, it is often taken at face value. Hence the misconception that NATO is milder ammo.
I am often asked how to convert CUP units to PSI. It cannot really be done. Not only is the measureing equipment different, but the place the pressure is measured varies from caliber to caliber. In a given caliber, one sytem might measure pressure at the neck whereas the other measures pressure in the middle of the case . In some calibers, the limits are higher or lower than the other system, and for a few the limits happen to be the same numbers. You can see the pressure limits under both systems at:
http://kwk.us/pressures.html
Note that they show the 308 Win limits as 62,000 psi and 52,000 CUP.
Because the location of the pressure point varies from cartridge to cartridge one cannot jump to conclusions that,say, a 308 is loaded hotter than a 30-06 or a 243. Each is measured in its own way, which may make pressure readings different, even though the actual pressure readings are the same.
Here is a page from another book where the same ammo was tested by both systems. As you can see, the CUP numbers are smaller, even though the pressures are the same.
I do agree that the NATO chamber can be deeper than a 308. This can lead to head separation if the case is full length sized all the way. Best procedure is to neck size only or FL size only enough to permit chambering.
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