Sister Cartridges Part 3: Pressure Prelude

Short Story: There are different ways to measure chamber pressure, and they are not all equivalent. There are different standards for chamber pressure, and they are not all equivalent.

We have already said that the argument of sister cartridge compatibility boils down to pressure. Specifically, chamber pressure.

In an effort to ensure the safety of manufactured cartridges, different organizations have embraced different testing methods and standards for measuring pressure generated by a cartridge. These standards apply to both civilian and military ammunition, and apply to the use of the cartridges in the appropriate chambers (e.g. 5.56 NATO cartridge in a 5.56 NATO chamber).

Unfortunately, there is a lot of misinformation out there when it comes to sister cartridges and pressure. Some of it is half truth. Some of it is outdated (by decades). Some of it is taken out of context. Some of it ignores the apples versus oranges nature of the testing. To be fair, the current information is not always accessible or easy to find, and when you find it, you need to know what you are looking at to make sense of it (e.g. even the U.S. Military doesn’t understand units). We will do our best to provide you with the best information that we have available to us.

Standardizing Organizations

• Military
  • U.S. Military
  • NATO (North Atlantic Treaty Organization)
• Civilian
  • SAAMI (Sporting Arms and Ammunition Manufacturers Institute)
    • This is the primary organization in the U.S. for the maintenance of standards for performance, interchangeability, and safety for civilian firearms and ammunition.
  • C.I.P (commission internationale permanente pour l’épreuve des armes à feu portatives)
    • This is the European equivalent to SAAMI.

Pressure Measurement Methods

Channel Sensor

Principle:

The test barrel chamber will allow installation of the channel sensor either at the case body (C.I.P.) or case mouth (EPVAT). The sensor is exposed directly to the gases when the cartridge is fired. For C.I.P., the case must be drilled in a precise location, to expose the sensor to the gas in the cartridge when fired. For EPVAT, the sensor is exposed to the gas once the bullet is dislodged from the case neck during the firing sequence. The gas pressure exposed to the transducer presses on a diaphragm, which transfers to the piezoelectric transducer, which produces a very slight electrical signal, which is amplified and translated into pressure by the instrumentation.

The resulting value is a direct measurement of gas pressure in the cartridge (C.I.P.) or chamber throat (EPVAT).

Additional Reading:

• The Ballistic Assistant: “Ways to Measure Pressure”

Conformal Sensor

Principle:

The test barrel chamber will allow installation of the conformal sensor against the case body. The piezoelectric transducer for a conformal pressure test has a contoured saddle that matches the cross sectional profile of the case body and is pressed against the case before firing the cartridge. The transducer is able to convert the deformation of the cartridge when fired into a very slight electrical signal, which is amplified and translated into pressure by the instrumentation.

The resulting value is an indirect measurement of gas pressure in the cartridge.

Additional Reading:

• Guns and Ammo: “Ammunition Pressure Testing”
• The Ballistic Assistant: “Ways to Measure Pressure”

Copper Crush Cylinder

Principle:

Copper crush cylinder testing involves installation of a pressure crush apparatus in a hole drilled in the chamber. An appropriately sized cylinder of copper is installed into the apparatus. When the cartridge is fired, the case bursts at the location of the sensor, which allows the gas to escape from the case into the apparatus. The gas pressure actuates the apparatus piston, which compresses the copper cylinder against an anvil. The amount of shortening of the cylinder corresponds with a specific pressure for that cylinder. The cylinder length is measured and compared against a reference table to estimate the actual pressure.

The resulting value is an indirect measurement of gas pressure in the cartridge.

It should be noted that the measurement using the copper crush method does not reflect peak pressures, unlike piezoelectric transducer measurements. As such, copper crush pressure will be numerically lower than the value generated by a transducer.

It should also be noted that the copper crush method does not plot pressure over time. It is a single data point per test run.

Additional Reading:

• Guns and Ammo: “Ammunition Pressure Testing”

Comparison of Methods and Conversion of Units

We must be careful when looking at the values from different methods. Some units cannot be converted to others, and test methods are not necessarily equivalent.

• Bar, psi, and MPa can be accurately converted to one another without any loss of fidelity.
• CUP cannot be converted to Bar, psi, or MPa (or vice versa). While CUP tends to be mildly proportional to other systems of measurement, there is no reliable regression between CUP and other units of measure.
• SCATP and SAAMI (piezo) use the same testing protocol and can be considered equivalent.
• C.I.P. is not equivalent to SCATP/SAAMI (piezo). C.I.P. is direct measurement of gas pressure, while SCATP/SAAMI (piezo) are indirect measurements.
• C.I.P. is not equivalent to Copper Crush Cylinder Test. C.I.P. is direct measurement of gas pressure, while Copper Crush is indirect. And the two tests have completely different units.
• EPVAT is not equivalent to SCATP/SAAMI, C.I.P., or Copper Crush. It is a completely different measurement location and cannot be translated to pressures measured in other locations.

Sister Caliber Standards

Maximum Average Pressure

Maximum Average Pressure, or MAP, is the basic average maximum pressure of a string of rounds (typically 10 rounds, but could also be based on AQL calculations). MAP is the pressure that is typically reflected in testing standards.

It is generally accepted that the MAP for a full lot of ammunition (i.e. every round tested) will be measurably higher than the MAP for a sample (e.g. 10 rounds). The expected lot average, called the Maximum Probably Lot Mean (MPLM), for a batch of ammunition is estimated based on the variation seen in the 10 round sample. MPLM is estimated to be MAP plus 2 times the standard deviation of the MAP (i.e. MPLM = MAP + 2*SD(MAP)). It is suggested (by SAAMI) that the 2 times the standard deviation of the 10 round MAP will be about 2.5% of the MAP. So the MPLM will be about equal to 1.025 * MAP. This figure does not typically make its way into standards, but you should be aware of the difference, in case you come across the term; MPLM is not MAP.

A Note About “Standards”

It should be noted that the current version of military standards are typically classified and restricted to military firearm manufacturers and personnel.

It should also be noted that standards change over time. The standards sometimes change acceptable pressures. For example, MIL-C-63989C for the M855 cartridge in 1994 indicated a SCATP MAP of 55,000psi. In 2001, Amendment 4 changed the SCATP MAP to 58,700 psi. While this doesn’t change the chamber, it can mean that a cartridge manufactured at one point in time may be loaded to a different pressure than a cartridge manufactured at another point in time.

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