850 kg/cm2 proof was commonly used for entry level Basque 12/70 guns of the 50's through the early 70's. Better grades carried 900 or 1000 kg/cm2 proof with some 12/70 pigeon guns 1200 kg/cm2.
The CIP standards were adopted by Spain some time after 1969.
Under the Belgian Proof House revisions of 1924
16g and 12g guns were proved at 900 kg/cm2 for a 600 kg/cm2 = 8534 psi maximum SERVICE pressure;
20g were proved at 1000 kg/cm2 for a 670 kg/cm2 = 9530 psi maximum SERVICE pressure
+ 10 - 14% by piezoelectric transducer measurement
That would be a service of 2/3 of proof.
850 kg/cm2 proof = 12,090 psi for a service pressure of 566.6 kg/cm2 or 8060 psi BUT as measured by lead crushers, so by modern piezo transducers the pressure would be + 10 - 14% or about 9000 psi
900 kg/cm2 proof = 12,801 for a 600 kg/cm2 = 8534 psi service pressure + 10 – 14% or about 9500 psi
1000 kg/cm2 proof = 14,223 psi for a service load of 9,473 psi + 10 – 14% or about 10,500 psi
1200 kg/cm2 proof = 17,068 psi for a service load of 800 kg/cm2 = 11,380 + 10 – 14% or about 12,500 psi
Those numbers are very similar to the British 3 tons = 850 kg/cm2 and 3 1/4 tons = 900 kg/cm2
850 kg/cm2 proof is equivalent to 3 tons service = 8938 psi (by transducers so 10 - 14% is not added)
900 kg/cm2 is 3 1/4 tons = 9682 psi
1000 kg/cm2 is 3 1/2 tons = 10,427 psi
1200 kg/cm2 is 4 tons = 11,917 psi
