Miller, I don't think the tons figures were ever meant to be converted by multiplying even Brit tons x 3, 3 1/4, etc. As you point out, that's way off. With crusher figures, inside the normal range of shotshell pressures, LUP + 1,000 will get you much closer--and will usually be somewhat on the conservative side. For example, the British crusher service pressure for an 850 bar proof gun was 650 bars. Multiply by 14.5 and you get 9,425 LUP. Adding 1,000 gets you quite close to the figure of 10,730 psi provided by the Brits--with a little bit of a margin for error built in. While it's certainly not exact, it's also unlikely to get you into trouble.
I recall seeing something on conversion years ago, but only vaguely. But as the numbers go up--for example, when you're talking proof pressure rather than service pressure--the difference, as I recall, is significantly greater than 1,000 psi.
For example, the previous magnum proof/current superior proof crusher figure was 1200 bar. That converts to 17,400 LUP. The correct psi figure is 2,000+ higher than that.
