You can use 14.5 for a conversion from BAR to PSI. Not sure is absolute but close enough will not get you in trouble. Thus 850BAR x 14.5 = 12,325psi "Proof" pressure. 650BAR x 14.5 = 9,425PSI Service pressure. Burrard reported on some pressure readings taken with a lead crusher & Piezo Electric gage simultaneously. Within ordinary shotgun pressures a "Workable" formula was devised. 1.5crusherP -.5 = PEP. Thus (1.5 x 3tons/sqin)-.5 = 4tons tons/sqin. Since the British do use the "Long Ton then 4 x 2240 = "Approximately" 8960PSI Service pressure for a 3 ton proofed gun.
Note the similarity here to the 650 BAR service pressure.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra