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GUNS Magazine Digital May 2011 - Page 24
• J O H N b A R S N E S S • THE MYSTERIES Of Pressure standards How changes in measuring procedures have caused some confusion. any handloaders wonder why the pressure standards M put forth by the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) vary so much between similar rifle cartridges. In some instances the answer is pretty straightforward and sometimes it’s not, but knowing require expensive pressure barrels. the reasons can help us be safer handloaders, as well as No matter the method used, SAAMI pressure tests must be put together consistent home-grown ammunition. According to SAAMI’s website (www.saami.org), the organization was founded in 1926 “at the request of the federal government” as part of the American National Standards Institute. Their mission is “to create and promulgate technical, performance and safety standards in firearms, ammunition and components.” Before SAAMI, different firearms companies used whatever dimensions they wanted for chambers and barrels, and ammunition companies could load rounds to whatever pressures they thought might sell. This could not only cause safety problems, but performance sometimes suffered. There are actually three distinct SAAMI pressure levels for each cartridge. The first is the maximum average pressure (MAP). Some public handloading data from some companies lists this number, even though it can confuse the average handloader. Part of the confusion is due to whether the MAP is measured by copper crusher or electronic transducer. Crusher testing is the older form of pressure measurement, first appearing in the 1860s, long before smokeless powder. In smokeless-powder testing, pressure from the test barrel’s chamber crushes a copper cylinder slightly. Measuring the amount of crush results in a reading, expressed as copper units of pressure (CUP). Electronic transducer testing was developed in the 1920s, but didn’t become common in the firearms industry until half a century later. Here, pressure from the test chamber results in an electronic impulse that varies in strength. The result is considered an accurate reflection of actual chamber pressure, so is expressed in pounds per square inch (PSI). Electronic-strain gauge testing is also becoming common, as it doesn’t Strain gauges are used in some modern pressure testing. compared to the same equipment’s results with SAAMI “reference ammunition,” ammo that’s been tested in standardized test barrels. Otherwise the readings from crusher, transducer or strain-gauge testing are just numbers, without any real meaning. standardization The industry is moving more toward transducer and strain-gauge testing, partly because a pressure reading immediately shows up on a computer screen, while in the crusher system the copper cylinder must be removed from the test barrel and measured. However, all three systems are still used, partly because some labs have a big collection of crusher barrels. Some of the confusion among handloaders arises from coppercrusher testing being used exclusively for so many years. The results were originally expressed in PSI, even though there’s a significant difference between CUP and actual pressure. The MAP for the .30-06, for instance, is 50,000 CUP and 60,000 PSI. These numbers express the same pressure, but they’re obviously different. In fact, CUP and PSI are so different that there isn’t an exact ratio between them, though a fairly complex formula can be used to convert between the two systems. Thus, shooters who started handloading before actual PSI became the common listing sometimes assume that CUP data for the .30-06 SAAMI pressure standards are designed to prevent “accidents” like these: leaky primer, brass flow into the ejector hole of the bolt face and blown primers. 24 WWW.GUNSMAGAZINE.COM • MAY 2011