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Art, chamber pressure was measured using copper (or lead) units of pressure decades before piezo or strain types of electric gauges were used starting in about 1970. Electric gauges were used in other industries beginning in the 1950's but rarely then in ammo testing. Factory and military specs were still based on copper units for sometime after 1970 'cause that's the system they used, trusted and already had plus it worked very well for all safety issues. But the vast majority of them stated the numbers in print as "psi" value when in fact it was copper (lead) units of pressure or CUP (LUP). When comparing numbers of both system, such as between the commercial .308 Win. using electric systems stating 62,000 psi or copper crusher systems stating 52,000 with both cup and psi suffixes compared to the military 7.62 NATO crusher system stating 50,000 psi, the confusion and mistakes began. They still exist 'cause so many folks don't communicate nor understand the realities of each.
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