Caution: Do not use the Western Powders load data for 5.56×45 without a lot of careful consideration
I was very surprised to see the separate 5.56 loads in the Western load data. That data constitutes a significant misunderstanding. After seeing it, I called Western Powders yesterday afternoon and explained what I am about to explain here. The response of the technician I spoke with was “Oh”; and then, “I’ll pass that along.”
There is no difference in absolute pressure between .223 Remington and 5.56 NATO. None! The two different Maximum Average Pressure (MAP) standards are artifacts of the different measuring instrumentation used here and in the CIP. The same lot of reference cartridges put in a SAAMI or a Lake City conformal transducer that gives readings averaging 55,000 psi will give readings averaging 62,366 psi in European channel transducers. This why the U.S. military and SAAMI specs have a maximum average pressure (MAP) of 55,000 psi and the Europeans have a MAP of 62,366 psi (actually, 4300 bar in their units). The European EVPAT 7.62 and 5.56 test procedures are based on results of their measurements of reference ammunition made in the U.S. using the U.S. test procedures, SCATP 7.62 and 5.56 for NATO compatible ammunition.
From MIL-C-9963F, for CARTRIDGE, 5.56MM, BALL, M193:
Quote:
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.
3.8 port pressure.
3.8.1 Measurement by copper-crush cylinder. -The average port pressure of the sample cartridges, conditioned at 70° ± 2°F, shall be 15,000 PSI ± 2000 PSI.
3.8.2 Measurement by piezoelectric transducer. -The average port pressure of the sample cartridges, conditioned at 70° ± 2°F, shall be 14,400 PSI ± 2000 PSI.
The piezo transducer in 3.7.2, above is a conformal piezoelectric transducer wherein the head of the piston that presses on the piezoelectric element is machined to match the chamber on the inside. This allows a cartridge to be placed into the chamber whole, and the pressure is measured over top of the brass.
The channel transducers used in Europe have a gas checked piston whose head is exposed directly to the propellant gas with no brass in between. The CIP does this by drilling a hole in the case and lining it up with the piston channel during firing, same as we do for copper crushers. The NATO EVPAT procedure samples the gas just in front of the case mouth to avoid the time consuming drilling and aligning.
The reason for the large size of the difference in readings between the two systems is not entirely clear. The brass interference does not account for it. The conformal transducer, for example, give 62,000 psi for .308 and 7.62 NATO ammunition that was 52,000 CUP in a copper crusher, while the channel transducers gives it only 4150 bar (60,191 psi), actually a lower number. The two systems are complicated by CIP having two different metric piston sizes, by masses not being the same, by electronic filtering being necessary to damp ringing in the EVPAT case mouth readings, which have their pressure applied more suddenly than through a drilled case, and selection of which filtering affects the reading values. It’s a very complex set of interactions and about the only thing you can say for sure is the systems don’t track one another.
The above is why reference ammunition is created. Even just within the SAAMI system, because your pressure tester won’t match mine exactly, even after calibration, one manufacturer is given responsibility for each chambering to load and supply reference ammunition to the industry. The reference cartridges are fired and whatever result you get on your system from them becomes the result you will use for your testing limit. Readings may be scaled to that as a correction factor.
In the case of the Western data for 5.56×45, the error is that they didn’t use channel transducers for the higher pressure limit. They used a conformal transducer. That ammunition would be expected to measure up to 4876 bar (70,719 psi) on the European equipment. Also, like most U.S. handloading data, no port pressure testing is done on the assumption the ammunition will be fired in bolt guns. It is not really NATO compatible ammo without gas port testing.
Will this hurt anything? Well, it’s not reaching proof pressures, which would be a minimum of 134% over MAP or 73,500 psi on a SAAMI conformal transducer. The CIP uses a lower percentage increase for proofing than SAAMI does, 125% over MAP, and that works out to 5375 bar (77,958) on a channel transducer. Brass will be stressed harder at this higher pressure, bolt face erosion and throat erosion will accelerate. Gas gun cycling will be harder.
It is interesting to note that when copper crushers were still in use in the original development and specifications for the cartridge, everything was much closer. This is because the way copper crushers were used by the various organizations was more similar. The copper crusher standards for M193 NATO ball ammunition are:
U.S. military: 52,000 psi (CUP, in SAAMI terminology, as this is by copper crusher)
NATO: 3700 bar (53,366 CUP)
The difference is so small as to be statistically insignificant, given the limited precision of copper crushers. Copper crushers have been phased out of military ammunition making, but the numbers are still valid standards and have not been changed.
The commercial .223 Remington copper crusher standards are:
SAAMI: 52,000 CUP
CIP: 3700 bar (53,366 CUP)
The SAAMI standard carried the U.S. military copper crusher MAP into commercial .223 loads, and the CIP standard carried the NATO copper crusher MAP over to commercial use. The European copper crushers use a slightly different pressure port location and a metric size piston, so the readings would not be expected to be exactly the same, even if copper crushers were perfectly repeatable.
Several years ago I had a phone conversation on the general topic of pressure standards with then SAAMI Technical Director, Ken Green (ret. 2011). The specific subject then was the MAPs for .357 and .44 Magnum cartridges. The original MAPs had been 45,000 CUP and 40,000 CUP for the two cartridges, respectively. But in the conformal Piezo transducer they are rated and 35,000 psi, and 36,000 psi, respectively, a decrease of 10,000 units for the .357 Mag, and if 4,000 units for the .44 Mag. Most every Internet pundit seemed to think this meant the two cartridges had been wimped down from their original pressures over liability concerns. Ken Green told me otherwise. He said the exact same reference loads made up in and used for copper crushers were fired in a Piezo transducer simply produced those different results. The two types of instrumentation just don’t track. Most people would like to believe absolute pressure numbers from these measurements were more exact than they actually are, but SAAMI’s own documentation show the same lots of reference ammunition can give results that vary over 23% in copper crushers and 11% in piezo transducers. This is why the reference ammunition is still necessary to keep people on the same page.
The original development of pressure levels for these rounds was done (see Elmer Keith’s work) until the gun being used said “uncle”, and then were backed off a percentage for a safety margin. They were not developed to an instrumentation standard, but to a gun. The instrumentation was applied subsequently to get a pressure standard that could be duplicated in manufacturing. If the absolute pressure standard had actually been lowered between the copper crusher and Piezo transducer standards, then the CUP’s would have been changed downward, too, and not left where they were. You don’t want a manufacturer still using a copper crusher producing different pressure ammunition than one using Piezo equipment. Again, those same copper crusher numbers are still in force, today. You can read the standards on SAAMI’s web site and see for yourself.
The same applies for the rifle rounds. The original pressures and reference loads remain the same. The Piezo numbers are just what SAAMI Piezo equipment reads them to be. It is the same with the CIP. Their copper crusher numbers remain valid, even though they are considered obsolete, because they represent results from the same reference ammunition. So, an unfortunate result is we have close to matching .223 and 5.56 pressures on everybody’s copper crushers, but not on the different piezo transducers.
