Originally Posted by KY Jon
Maybe we ought to make this thread, or one like, it a sticky. One of the most asked and often misunderstood question, is what load to shoot in my gun? A gun in proof, which has been inspected by a knowledgeable gunsmith, should be safe shooting a “service pressure” load of 740 bars which is 10,732.79 psi. For additional margin of safety you can always go lower, as I do. A Black Powder proofed gun gets loads comparable to what was original to it, and those are 4,500-5,500psi. Or if you can load them, real black powder loads which always draws a lot of attention.

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Jon, there's an issue with the bars figures you use. Back when the Brits stamped their guns 850s bar proof, the service pressure figure they used was 650 bars service pressure. BOTH of those figures were derived by the old lead crusher system. You correctly converted 650 bars crusher to 740 bars transducer so that it could then be converted to psi using the mathematical formula. 850 bar is also a crusher-derived figure. Thus, it also needs to be converted. The proof pressure for an 850 bar gun is 960 bars transducer, which converts to 13,920 psi proof pressure. That's straight from Roger Hancox, the former Birmingham proof master. But as you point out, the 10,730 psi service pressure is the number that's really important as we work up reloads for guns marked 850 bar, or the current "STD".