I posted this as a response to an article posted on the Battle Rifle Forum. It might be of more use, here.
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 pressure difference, there would be a noticeable difference in velocity.
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 (commercial). 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 standard reference ammo (Winchester commercial) 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 piezoelectric 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 literature listed CUP pressures as psi, and that literature 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 measuring equipment different, but the place the pressure is measured varies from caliber to caliber. In a given caliber, one system 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.
In the context of milsurp rifles, what does all this mean?
There are two issues. Case construction and pressures.
Some rifles, and the M14 and FAL are two good examples, do not support the entire case head all the way back to the extractor groove edge. The web part of the case head, the solid brass section at the bottom of the case, is intended to seal or plug any unsupported area. The web will hold the pressure - the case wall will not. If the unsuported area is so big it can uncover part of the case wall, the case will blow, casing total destruction of the rifle and perhaps injury. I have seen this once. Strangly enough, it involved a military case, which usually have thicker webs than commercial cases. This was Santa Barbara military (Spanish, I think) and when we sectioned some cases after tha accident we discovered the lowest web we had ever seen. Lower than any commercial.
The rifle was assembled out of spec. The chamber was a bit short, leaving too much case head unsupported. It never mattered untl the day it fired the Santa Barbara ammo.
If you shoot a military semi-auto, it would be a good idea to shoot Canadian or USA milsurp ammo in it, because of the higher (thicker) web. If considering some other type of milsurp, cut a case in half, first, to see how the web looks.
The second issue is pressure. Rifles originally designed for 308 or 7.62 can safely shoot either one (but a 308 Garand should be limited to lighter bullets/faster powders so as to not exceed the port pressure limit). A 98 Mauser, for example, handles 308 just fine.
But what about the older designs, like the 93, 95 and 96 Mausers and the Lee Enfield #4? They were intended for milder ammo, but in practice, they shoot 308 and 7.62 just fine. Hundreds, if not thousands of #4s and M96s were used in Commonwealth countries as target rifles, without a problem.
But, that said, be aware there is less safety room for excess pressure. Don't handload near the limit. Don't seat bullets touching the rifling. Watch for pressure signs. Each rifle is different. If your particular rifle is giving pressure signs, drop the load or try a differnt brand of ammo.
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 pressure difference, there would be a noticeable difference in velocity.
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 (commercial). 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 standard reference ammo (Winchester commercial) 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 piezoelectric 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 literature listed CUP pressures as psi, and that literature 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 measuring equipment different, but the place the pressure is measured varies from caliber to caliber. In a given caliber, one system 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.
In the context of milsurp rifles, what does all this mean?
There are two issues. Case construction and pressures.
Some rifles, and the M14 and FAL are two good examples, do not support the entire case head all the way back to the extractor groove edge. The web part of the case head, the solid brass section at the bottom of the case, is intended to seal or plug any unsupported area. The web will hold the pressure - the case wall will not. If the unsuported area is so big it can uncover part of the case wall, the case will blow, casing total destruction of the rifle and perhaps injury. I have seen this once. Strangly enough, it involved a military case, which usually have thicker webs than commercial cases. This was Santa Barbara military (Spanish, I think) and when we sectioned some cases after tha accident we discovered the lowest web we had ever seen. Lower than any commercial.
The rifle was assembled out of spec. The chamber was a bit short, leaving too much case head unsupported. It never mattered untl the day it fired the Santa Barbara ammo.
If you shoot a military semi-auto, it would be a good idea to shoot Canadian or USA milsurp ammo in it, because of the higher (thicker) web. If considering some other type of milsurp, cut a case in half, first, to see how the web looks.
The second issue is pressure. Rifles originally designed for 308 or 7.62 can safely shoot either one (but a 308 Garand should be limited to lighter bullets/faster powders so as to not exceed the port pressure limit). A 98 Mauser, for example, handles 308 just fine.
But what about the older designs, like the 93, 95 and 96 Mausers and the Lee Enfield #4? They were intended for milder ammo, but in practice, they shoot 308 and 7.62 just fine. Hundreds, if not thousands of #4s and M96s were used in Commonwealth countries as target rifles, without a problem.
But, that said, be aware there is less safety room for excess pressure. Don't handload near the limit. Don't seat bullets touching the rifling. Watch for pressure signs. Each rifle is different. If your particular rifle is giving pressure signs, drop the load or try a differnt brand of ammo.
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