Note: .223 Rem SAMMI load data is MAX @ 55Kpsi; 5.56mm is MAX @ 62Kpsi. Almost all published data by bullet and powder makers follows .223 MAX Pressure specifications. For example Nosler publishes online data for both the .223 Rem and 5.56mm. But the loads are identical.......but .223 Rem and 5.56 chambers and reamers are not the same. The 5.56 has a longer lead than the .223 Rem allowing higher pressures, but also remember brass life is not a concern for the military users of the 5.56mm.
BobinNC, Please do not take this the wrong way it is NOT meant to be an insult, my posting is about getting the correct information out to people so they understand pressure readings.
There are three methods of measuring chamber pressure, and they are all in pounds per square inch.
(its how we Americans read pressure in the U.S.)
1. The oldest is using a copper crusher pellet called "Copper Units Pressure" or CUP
2. The newer method is with a electronic transducer and it is labeled "PSI" to separate it form the copper crusher method.
3. The European method under CIP (NATO standard) uses the transducer method "BUT" it is located at the mouth of the cartridge case. (Under U.S. SAAMI standards the transducer is located at the mid-point of the case)
All three pressures listed below are the exact same pressure and are just measured three different ways.
1. 52,000 cup
2. 55,000 psi
3. 62,000 psi European CIP/NATO standard
Again ALL three pressures above are the exact same pressure.
Below is the milspec requirements for commercial contract ammunition for the military, again please note that 52,000 cup and 55,000 psi are the exact same pressure. And if the European CIP/NATO method was used it would read 62,000 psi.
MIL-C-9963F
15 October 1976
SUPERSEDING
MIL-C-9963E
12 May 1970
MILITARY SPECIFICATION
CARTRIDGE, 5.56MM, BALL, M193
3.7 Chamber pressure.
3.7.1 Measurement by copper-crush cylinder.-The average chamber pressure of the sample cartridges, conditioned at 70° ± 2°F, shall not exceed 52,000 pounds per square inch (PSI). The average chamber pressure plus three standard deviations of chamber pressure shall not exceed 58,000 PSI.
3.7.2 Measurement by piezoelectric transducer.-The average chamber
pressure of the sample cartridges, conditioned at 70° ± 2°F, shall not
exceed 55,000 PSI. The average chamber pressure plus three standard
deviations of chamber pressure shall not exceed 61,000 PSI.
What this means is very simple, the commercial .223 and the military 5.56 are both rated at the same pressure. BUT because the throats are different if you shoot a .223 in a 5.56 chamber the pressure will be lower than normal. And if you shoot a 5.56 in a SAAMI .223 chamber the pressure will be higher than normal.
Now to add even more confusion, my factory made Savage .223 with a 1 in 9 twist has a longer throat than my AR15 rifles and it is NOT a standard SAAMI throat. The SAAMI only sets basic guidelines and they do not have to be followed, what this means is I can safely shoot military ammunition in "MY" rifle but a short throated .223 will have higher pressures.