For some reason I got confused and have the pressures for 8 bores . All bores are proofed at the same pressure for that bore irispective of chamber lengths for instance 12 bore 2%1/2 " and 2&3/4 " are proofed at 950 bar for a max sevice load of 740 bar all 3" are now classed as magnum or "superior" and will be proofed at 1320 bar. 8 bores if new would be proofed at that ,however as they are considered in the UK as an obsolete caliber ans as there is no commercialy loaded cartridges available the Birmingham poorf house will re proof at 850/900 bar for a maximum service pressure of 680 bar .This is because most 8's would not stand the higher proof pressures ,not so much the barrels but the old actions are not strong enough to withstand the back pressre.
As 10 bore cratriges are available , 10's 3inch plus would be proofed at 1320 bar .
Please note that as they CIP have once again changed the metod of measuring the proof pressures it is not possible to make a direct traslation to other systems of measurement .
