So how much pressure is this?

Per the British 1954 Rules of Proof.......

3 tons proof = 8,938 PSI
3 1/4 tons proof = 9,682 PSI
3 1/2 tons proof = 10,427 PSI
4 tons proof = 11,917 PSI

The British proof house measures in Long tons per square inch.

Thank you, sir.

Note that to refer to the "tons" marks as proof is misleading. The psi figures given by PA24 are not proof pressures, but rather service pressures: max pressure for shells to be used in guns so marked.

How does this compare/what is the PSI conversion for Spanish guns marked 900kgs?
Thanks,
CHAZ

The British Ton = 2240 lbs, so 3 Tons = 6,720 lbs. As Larry said though the gun was proofed to a higher pressure & certified as proofed for 3 Ton pressure loads. The 3 ton reference was also a crusher pressure measurement & not the later PSI as obtained by use of Pieso-Electic crystals which will show a higher pressure than the crusher readings.

Incorrect.....

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One (1) British long ton axial CUP = 2638 PSI..CIP radial PSI....

British pressures are HIGHER than they seem, i.e. 1 long ton is only 2240 lbs, but one long ton PSI is 2638 PSI.....

The conversion of an axial base measurement to the equivalent radial measurement:

Conversion of radial crusher (CUP) to radial transducer (psi) measurement is well approximated by the following relation: psi = 9e-6 *CUP^2 + 0.7051 * CUP. Otherwise expressed as 9 times 10 to the -6 power times CUP. This conversion works well for most modern values of European and American pressure in firearms.....

Also note:

For the modern Piezo equipment ....1 British long ton = 2850 Piezo PSI.....
Therefore THREE (3) Long Tons would be 8550 PSI as measured by the modern Piezo equipment.....

http://en.wikipedia.org/wiki/Piezoelectricity

http://en.wikipedia.org/wiki/Commission_Internationale_Permanente_pour_l%27Epreuve_des_Armes_%C3%A0_Feu_Portatives

http://en.wikipedia.org/wiki/Birmingham_Proof_House

Re the conversion of bars/kg readings to psi: per the above, it gets somewhat complicated if the bars/kg are measured via the crusher system. You can't multiply by 14.5, which works if we're talking bars measured via piezo. Per the Birmingham Proofhouse, a gun with an 850 bar proof (which is a crusher measurement) has a service pressure (piezo) of 10,730 psi, which is slightly less than our American SAAMI standard of 11,500 psi for 12ga guns. 900 bar/kg crusher would thus indicate a service pressure essentially identical to our SAAMI standard.