Small arms ammunition pressure testing

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Small arms ammunition pressure testing is used to establish standards for maximum average peak pressures of chamberings, as well as determining the safety of particular loads for the purposes of new load development. In metallic cartridges, peak pressure can vary based on propellant used, primers used, charge weight, projectile type, projectile seating depth, neck tension, chamber throat/leade parameters. In shotshells, the primary factors are charge weight, projectile weight, wad type, hull construction, and crimp quality.

Modern civilian test methodologies[edit]

The two modern standardized test methodologies in use are the Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives or C.I.P. methodology, and the Sporting Arms and Ammunition Manufacturers' Institute or SAAMI methodology. The SAAMI methodology is widely used in the US, while C.I.P. is widely used in the mainly European C.I.P. member states. While both modern methodologies use piezo pressure transducer sensors to general pressure readings, differences in the test setup mean that the same pressures will often generate very different readings depending on the method used.

Because C.I.P. and SAAMI maximum pressures are measured differently, it is important to know which methodology was used to measure peak pressures. While C.I.P. pressures are often quoted in megapascals in Wikipedia and bars by C.I.P., and SAAMI in psi, it is not unusual to see C.I.P. pressures converted to psi or vice versa.[1]

C.I.P. method[edit]

Some have incorrectly concluded that C.I.P. measures pressure at the case mouth to account for the variations from SAAMI pressure.[1]

C.I.P. uses a drilled case to expose the pressure transducer directly to propellant gases. The piezo measuring device (transducer) is positioned at a distance of 25 millimetres (0.98 in) from the breech face when the length of the cartridge case permits that, including limits. When the length of the cartridge case is too short, pressure measurement will take place at a chambering specific defined shorter distance from the breech face depending on the dimensions of the case. The defined distance for a particular chambering is published in the TDCC data sheet of the chambering.
In a rifle cartridge case like the .308 Winchester, the TDCC M = 25.00 value denotes the transducer must be positioned at a distance of 25 millimetres (0.98 in) from the breech face.[2]
In a relatively short pistol cartridge case like the 9×19mm Parabellum (9mm Luger in C.I.P. nomenclature), the TDCC M = 12.50 value denotes the transducer must be positioned at a distance of 12.5 millimetres (0.49 in) from the breech face.[3]

As transducer C.I.P. almost exclusively uses one type of Piezoelectric sensor (named "channel sensor") made by the Swiss company Kistler that requires drilling of the cartridge case before firing the proofing cartridge in a specially made test barrel.[4][5][6] The measurement accuracy of the pressure measurements with 21st century transducers is expected to be ≤ 2%.[7]

SAAMI method[edit]

SAAMI uses a conformal piston method using another type of Piezoelectric sensor (named "conformal sensor") mostly made by the US company PCB Piezotronics that does not require prior drilling of the cartridge case. Instead a small piston is inserted in a hole drilled in the chamber to rest against the brass, with the piezo transducer on the other side of the piston. This has the benefit of protecting the transducer from hot gasses, but the drawback of creating artifacts in the data due to the wall of the brass often failing and bursting near 20,000 PSI.[1] Further the conformal sensors are more expensive to use, since each chambering needs its special transducer.

Shotshell ammunition method[edit]

For shotshell ammunition, the technical variations are easier to solve since only one type of Piezoelectric sensor (called "tangential sensor") is available from the PCB Piezotronics and Kistler International companies to be used without drilling without variations amongst SAAMI guidelines and C.I.P. rulings.[8]

External stress gauge[edit]

A low cost method of gathering pressure data uses a resistive stress gauge attached to the exterior of the chamber. These systems are usually calibrated to emulate the results of an existing standardized system such as the SAAMI system, so that the results will be directly comparable. Because this system does not require a specialized test barrel, only a firearm that has an accessible external chamber wall, it is much lower cost.

Military test methodologies[edit]

The US military defines test procedures for 5.56mm NATO in SCATP 5.56, and 7.62mm NATO in SCATP 7.62.[9] These procedures are based on the SAAMI test methodology.

NATO, however, defines 5.56mm, 7.62mm, 9mm and 12.7mm using the NATO EPVAT test methods, which includes pressure testing. Unlike the civilian testing methods NATO EPVAT testing procedures for the "NATO rifle chamberings" require the pressure sensor or transducer to be mounted ahead of the case mouth. The advantage of this mounting position is that there is no need to drill the cartridge case to mount the transducer. Drilling prior to firing is always a time consuming process (fast quality control and feedback to production is essential during the ammunition manufacturing process). The disadvantage of this mount is that the pressure rises much faster than in a drilled cartridge case. This causes high frequency oscillations of the pressure sensor (approx 200 kHz for a Kistler 6215 transducer) and this requires electronic filtering with the drawback that filtering also affects the lower harmonics where a peak is found causing a slight error in the measurement. This slight error is not always well mastered and this causes a lot of discussion about the filter order, cutoff frequency and its type (Bessel or Butterworth).[10] For the 9mm NATO EPVAT specifies that for 9×19mm Parabellum (9mm Luger in C.I.P. nomenclature and 9mm NATO in NATO nomenclature), the transducer must be positioned at the mid case position (9.5 millimetres (0.37 in)) from the breech face instead of C.I.P.'s 12.5 millimetres (0.49 in) from the breech face. For NATO EPVAT testing of military firearms ammunition NATO design EPVAT test barrels with Kistler 6215 channel sensor transducers are used.[11]

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