HOS, I think you have the horse and the cart reversed. The British proofhouses no longer offer a range of proofs. You did find different bars markings when those were used (until 2005). In part, that was to accommodate the fact that small bores are proofed to slightly higher pressures than 12's. (Same deal with SAAMI service pressure standards in this country. 12's and 16's are 11,500 psi; 20's, 12,000 psi; 28's and .410's, 12,500.) So the proofhouses had to make allowance for those differences. For example, I recently saw a vintage British 20ga with chambers lengthened to 2 3/4" and reproofed at 950 bars.
However, the bars are now out (as of 2005), and they have been replaced by 3 levels of proof, or actually only two with a variation attached to one. Those are marked STD for standard (which replaces the old 850 bars for 12's, or slightly higher for smaller bores); and SUP for superior, which replaces the old 1200 bar magnum proof. There is also now a steel shot proof, which is the same as the superior proof with the exception that the proof load uses steel rather than lead shot. And that's it. The gunmaker could request any of those three levels of proof with a 2 3/4" or longer gun. If he were to build a new 2 1/2" gun, or if he were to build a very light game gun with 2 3/4" chambers, then he'd request the standard proof. Of course they all try to build their guns so that there's some built-in cushion above that to which it will be subjected at proof.
And just as SAAMI does in this country, the CIP sets pressure standards for ammunition manufactured in the UK and European countries. If the ammo makers want the CIP seal on their ammo, then they have to comply with the CIP standards. And the shell box will specify the proof level of the guns appropriate for the shells in question. I have yet to see any boxes marked with STD or SUP, but I expect some of our British posters have.
