One other factor you have to remember in this quite complicated proof/pressure business: the "tons" markings were for service pressure. In other words, if 3 tons equates to 8900 psi, that's the upper pressure limit of the loads you should be shooting in the gun. The more recent "bars" markings are proof pressures. So, for example, you can't take 850 bars--current CIP standard proof pressure--multiply it by 14.5, and arrive at a pressure limit for your loads. (And you can't do that for another reason anyhow, at least on Brit guns, which is that the "bars" in question are crusher bars, not piezo bars--which means the resulting figure is LUP, not psi.)
It can make your head spin.
