The problem here, as noted above, is like the old remark (George Bernard Shaw?) about the Americans and the British being two peoples separated by a common language. Thus it is with proof, service pressure, etc.
As Ian noted, although the British now express proof pressure in bars (since the late 80's), you cannot do the bar x 14.5=psi, because the Brits don't use the more modern electric transducer system, but rather the old lead crusher system--which did not accurately convert to psi. Straight from the Birmingham Proof House, which is the most reliable source I know of when it comes to questions about proof, service pressure etc on British guns:
"The equivalent transducer values that should be used for comparison with SAAMI transducer values are 740 bar service and 960 bar proof, i.e. 10,730 psi (service) and 13,920 psi (proof)."
If you look in older reloading manuals (or even some newer ones), you'll find the pressure of various reloading formulas expressed in LUP (or lead units of pressure). Those are measurements derived from the old lead crusher system, which the British still use. There's no formula to convert crusher values to transducer values, but an acceptable "ballpark" conversion is: LUP + 1,000 = psi. (Which you can also reverse, if for some reason you want to convert from psi to LUP.) If you add 1,000 to the figures Miller gives above, you come pretty close--especially to service pressure.
Mike, if your gun has 850 bar stamped on it, then it was submitted for reproof some time since the late 80's. (You may even find a 2-digit year of reproof, if you examine the barrel flats carefully.)
Regardless of the pressures reported on American factory loads, I would not use them in that gun. SAAMI service pressure for the 12ga is 11,500 psi. One reason American ammo makers don't state pressure on their shell boxes is because they load to a particular velocity level, not a pressure level. That is, they may very well change powder from one lot to another, and while the velocity of the loads will remain quite constant, the pressure will not. As long as they stay within the SAAMI standard, they don't concern themselves with pressure. CIP (British/European) shells are loaded to conform with the lower CIP pressure standards, which you can be sure will not exceed the service pressure of an 850 bar proof gun. So your choices are either CIP shells (or their American equivalent, from companies like RST and Polywad), or low pressure reloads. Because it's so easy (especially with the 12ga) to keep reloads under 9,000 psi--even for 1 1/8 oz hunting loads--that's the route taken by many people who shoot older doubles proofed to lower pressure standards.
