Ammunition Pressure Testing

Most commercial ammunition manufacturers have standards used for loading ammunition that include testing and measuring what the pressure inside the cartridge case is when a round is fired. In actuality, manufacturers that are members of the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) spend huge amounts of time and money to ensure that their ammunition is loaded to accepted specifications for pressure performance. These specifications encompass the equipment used, techniques used and a common set of pressure standards for ammunition. In today’s age of rampant lawsuits, it is important for these manufacturers to ensure that they use accepted practices and techniques to control the manufacture of ammunition while producing consistent and safe products.

I thought it might be interesting to share some of these standards, and highlight the equipment and procedures used by the industry for measuring pressure.

Pressure Standards

The most important number manufacturers adhere to when loading ammunition is the Maximum Average Pressure (MAP). This is the highest average pressure allowed in a group of test ammunition during a production control test. Manufacturers generally design loads to stay somewhat below this number in order to account for component variations and ensure that, on the whole, the products maintain a consistent velocity level in line with the MAP. For example, the .30-’06 Springfield has a piezo MAP of 60,000 psi. A 150-grain bullet in the .30-’06 has a nominal specification of 2,900 feet per second (fps). Most manufacturers will try to select a powder load that will produce this velocity at, say, 56,000 psi in order to allow some room for a fast lot of powder, a lot of cases with slightly smaller case capacity or high neck tension. Even with these variables, they would still be able to achieve the 2,900 fps mark within the 60,000 psi MAP.

SAAMI also defines Maximum Probable Sample Mean (MPSM) and Maximum Probable Lot Mean (MPLM) pressures. These values are derived statistically from the MAP and are really not used to control the manufacture of ammunition. The MPSM is probably the most meaningful of these to a ballistician. It basically means that across an entire lot of ammunition, a randomly selected sample of ammunition that tested within the MPSM parameters would be considered within specification. For our .30-’06 example, the MPSM is 63,800 psi. The SAAMI website has a complete description and the mathematics behind these numbers for those interested.

Pressure Measurements

There are four recognized ways to measure pressure in small-arms ammunition. The oldest, and largely unused today, is the copper and lead crusher method. There are three distinct piezo methods used: The SAAMI conformal transducer method; the CIP drilled case direct read transducer method; and the NATO EPVAT (Electronic Pressure Velocity and Action Time) case-mouth direct-read system.

Copper/Lead Crusher Method

Before the advent of sophisticated electronics, a method was developed to use precisely made copper and lead cylinders to measure the relative amount of pressure a round of ammunition produced by how much the cylinder of metal was shortened, or “crushed,” when it was exposed to the internal pressure in a cartridge case. It has been used, as far as I know, since before the last century, and has been almost entirely replaced by the piezoelectric method. Several different size copper-crusher cylinders are used to measure pressure depending on the pressure range of the cartridge. A small-diameter cylinder is used for lower-pressure pistol cartridges because it will deform more and provide better pressure resolution at lower pressures. A larger copper cylinder is used for higher pressure pistol and rifle cartridges. Lead crushers were primarily used for blackpowder shotshell loads because of their low pressure and the need for a metal that deformed more easily than copper. Lead crushers have not been used since probably the 1960s. The metal cylinders are calibrated by testing them with precisely known static loads and measuring their deformation. Each lot of crusher cylinders is supplied with a table of Copper Units of Pressure (CUP) versus crushed cylinder length. The crusher method has several drawbacks: It does not give the absolute or true peak pressure of the round, but it gives CUP or Lead Units of Pressure (LUP). It does not give any sort of pressure versus time data. It is also a tedious and slow process for conducting pressure testing.

The actual test method for using copper crushers is quite simple. The pressure-test barrel has a hole drilled into the chamber. A gas check is inserted first into the hole followed by a piston with a large head on it. The copper crusher is placed on top of the piston and is trapped between the piston and an anvil attached to the test-barrel’s receiver. When the round is fired, the pressure blows a hole in the side of the cartridge case, and the gas pressure drives the piston upward compressing the copper crusher cylinder between the piston and anvil. The crusher cylinder is removed and then measured for length. This compressed length is then referenced on the table of CUP versus crusher length, and the maximum pressure for that round is recorded in CUP.

Because of the relatively slow response time of the copper crusher cylinders, they are not capable of measuring a rapid-pressure event to a precise, absolute value. To explain this, the response time of a copper crusher cylinder is somewhere on the order of milliseconds, where the response time of a piezoelectric transducer is on the order of microseconds — or 1,000 times faster. When you consider that small arms ammunition action times are roughly 1 to 3 milliseconds, you can see why the crusher method is not capable of measuring an absolute pressure of an event this fast. Nonetheless, crushers provide measurement of pressure for controlling the production of ammunition to a safe and consistent pressure level. The slow response time of the metal crushers, coupled with the energy lost to piercing the case and compressing the crusher, explains why we always see the difference in pressure specifications between CUP and PSI. For example, our .30-’06 has a copper-crusher pressure MAP of 50,000 CUP and a piezo pressure MAP of 60,000 psi.

Piezoelectric Method

Piezoelectric-pressure-measurement techniques, or “piezo,” replace the metal cylinder with a crystal that, when compressed or stressed, gives off an electric charge that is linearly comparable to the pressure it is being exposed to. The crystal is enclosed inside a transducer and screwed into a test barrel where it experiences the internal pressure in the chamber. The output from the transducer is fed through an amplifier where the charge is amplified and recorded. An actual pressure-versus-time plot is near-instantly available with the piezo method. The units associated with any piezo pressure measurement is an actual gauge-pressure in units of pounds per square inch, megapascals, etc. Piezo pressure transducers became generally available in the 1960s.

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Piezo transducers have to be calibrated to determine their pressure-versus-output relationship. This is done by using a high-pressure hydraulic pump with its own calibrated transducer, or analog dial pressure gauge, to expose the uncalibrated transducer to a range of known pressures in order to calculate the output of the transducer versus the input pressure.

There are two types of transducers used in small arms ballistics. The first is the direct-read transducer. These transducers are directly exposed to the chamber pressure via a small hole drilled through the side of the chamber wall and a hole in the cartridge case. This method provides very high-fidelity measurement of the entire interior ballistics event, including the primer output and propellant ignition. This is the method used by CIP, the international ballistics standard used everywhere except the United States. This method has several drawbacks. The transducer has to be protected from the high heat in the chamber or measuring errors will occur. This can be accomplished by coating the sensing surface of the transducer with a ceramic coating and/or coating the sensing surface with silicone grease. This method is also inconvenient because loaded rounds of ammunition have to have a small hole drilled in them. This hole has to be carefully aligned with the hole in the chamber for the transducer when the case is loaded. The process is a bit tedious and slow.

The NATO EPVAT system also uses a direct-read transducer, but solves the drilled-case problem by measuring the pressure at the case mouth. The transducer still has to be protected from heat. Another drawback of the case mouth method is that the pressure hole is drilled into the lead of the rifling. During testing, this method has a tendency to shave a small amount of jacket material from the bullet that must be cleaned out of the pressure hole.

In the 1980s, an ingenious piezo transducer was developed by PCB Piezotronics. PCB developed the conformal piezo transducer that placed the crystal inside the usual transducer, but cut the face of the transducer to match the radius and taper of the specific chambering it was designed for at a point just behind the shoulder body junction. The pressure is actually measured through the case wall. Once enough pressure is produced inside the case to cause it to swell and touch the chamber walls, the conformal transducer accurately measures the chamber pressure. This technique allows the test ammunition to be loaded and fired just as in a firearm, and the speed with which ammunition can be fired is only limited by how fast the test barrel can be reloaded. This is the pressure-measurement method used across the board by SAAMI-member manufacturers.

Conformal transducers are calibrated in a fixture, called a “calibration adapter,” which is the same dimensions as a minimum cartridge chamber and attaches to the hydraulic calibration pump. It accepts both the transducer and a cartridge case. Each lot of cartridge cases has to be calibrated to the transducer. The output of the transducer is still linear, but there is now a fixed-pressure offset that is added to the transducer pressure. This offset pressure comes from the calibration, and how much pressure it takes to get that specific lot of cases to swell enough to touch the sides of the chamber, and therefore the transducer.

The conformal piezo method is very convenient and easy to use, but expensive. Rather than one transducer that can be used in every caliber, there is now an individual transducer and calibration adapter required for every cartridge. However, the speed and convenience of testing afforded by the conformal transducer far outweighs the expense of the hardware. The only real drawback of the conformal piezo method is that the early time ignition of the propellant cannot be observed because no data is recorded until the case touches the transducer. This is usually not an issue with testing meant to control the loading of ammunition. Propellant manufacturers will use direct-read drilled-case testing to investigate the early time performance and ignition of propellant.

Summarizing

I hope that after reading this and looking at the pictures, you have an understanding of how pressure is measured in small arms. From this point, it is a highly evolved system. In particular, the convenience and speed of the conformal piezo system makes it easy to measure pressure and perform cartridge and propellant development. In future reading, when you encounter discussions involving CUP or PSI, you’ll understand what’s going on.