Virginian, the previous crusher method of measuring pressure (lead crushers for lower pressures, like shotguns; copper crushers for higher pressures, like rifles) turned out to be inaccurate when we began measuring pressure with piezo-electronic transducers. Thus, older American pressure readings expressed as psi were inaccurate, as were British pressure readings expressed as bars. If you're familiar with older reloading manuals, you'll find at least some pressure readings expressed as LUP (lead units of pressure) rather than psi. LUP is simply shorthand for the inaccurate psi figures derived from crushers. You can get a ballpark psi figure by adding 1,000 to an LUP figure. When the Brits converted from their old "tons" proofmarks--and that one doesn't work either if you multiply by 2,000--they changed over to bars. But they were still using crushers, so if you used the bars x 14.5 formula, what you came out with was, in effect, an LUP reading rather than psi--or roughly 1,000 off.
Under the 1989 Rules of Proof, British standard proof pressure (marked on many guns from that period) was 850 bars. Service pressure was 650 bars. In an email from the Birmingham Proof House back in 2001, proof master Roger Hancox explained what had to be done to convert those figures to transducer numbers and come up with equivalent psi numbers: "The equivalent transducer values that should be used for comparison with SAAMI (US) transducer values are 740 bar service and 960 bar proof, i.e. 10,730 psi and 13,920 psi."
In other words, you've got to make sure you're talking about pressures measured by the modern (transducer) system before you use the standard formula to correctly convert bars to psi. Fortunately, under the latest Rules of Proof, the British (and all other European gunmaking countries) are now using transducer figures rather than crusher figures--but the Brits no longer use a proofmark with the bars figure. They've now gone to STD under a crown for standard proof (Roger Hancox's 960 bars/13,920 psi figures) or SUP under 2 crowns for superior proof. (That one is 1370 bars/19,365 psi.) And if you see pressure expressed in numbers, it will now be megapascals (abbreviated MPa). One MPa = 10 bars, so to convert MPa to psi, you would multiply by 145 instead of 14.5.
The Brits work hard at keeping us on our toes!
But at least they're now measuring pressure with transducers rather than crushers.
