I think Bro Larry has cleared up this murky mess and hope this is the 'final' word:
British proof markings: The "tons" mark (which is service pressure) was not used until 1954. Between 1925-54, the chamber length was marked. 1904-1925 guns would not have the chamber length, but only the shot charge and a nitro proof.
http://www.gunproof.com/Proofing/proofing.html Information about the Birmingham Proof House
http://www.basc.org.uk/media/100_proof.pdf There are three pressure measurement systems (lead crushers/LUP, copper crushers, and piezo), and two standard systems of measure units (USA/SAAMI and CIP/British/European) and two differing units (psi and tons and bar and Kg/Cm sq) within each system, different proof laws by country, and differing proof laws by age.
Unfortunately, there is essential NO formula to convert LUP (CIP) to PSI (SAAMI). LUP + 1000 does not reliably predict the actual piezoelectric transducer measured pressure.
In 1989, all proof data was marked in metric using the BAR as the unit of measurement. The use of kilograms per square centimetre was dropped. Under this system, 850 bar represented standard proof, with 1200 bar being special or magnum. 2 1/2" chambered guns are usually proofed 850 bar (3 tons per sq. inch)
2 3/4" chambered may be proofed 900 bar (3 1/4 tons per sq. inch) or higher.
**650 bars or 3 TONS is standard CIP (British and European) service pressure for shells designed for guns proofed at the CIP standard of 850 bars**
**A service pressure of 3 tons per sq. in. =European proof pressure of 850 kg per sq. cm. (BAR)= 13,920 psi proof pressure= 10,730 psi service pressure= 650 BAR service pressure**
For comparison, here's data from actual pressure testing:
3 Dr.Eq. 1 1/8 oz.
Winchester Trap Load1,202 fps 9,600 psi
Winchester AA Xtra-Lite 1 oz WAAL12
1189 fps 8000 psi
The 12g 2 5/8" 1 1/8 oz. 3 1/4 Dram Eq (1295 fps) load of the 1920s ran about 10,500 psi.
Baschieri & Pellagri data (keep in mind that 650 bar is the service (using) pressure for guns with proof of 850 bar) BUT you must add about 1000 to the reported pressure to get an approximation of the SAAMI equivalent PSI.
http://www.bandpusa.com/files/db_trio.pdf Gamebore Traditional Game 16g 28gm (1 oz.) is 1260 fps at 8122 psi. The box is labeled "These cartridges are suitable for use in:
70mm case length: Guns with a chamber length of 2 3/4" or longer, nitro proofed to a service pressure of 3 1/4 tons per square inch (900 kg per square cm)
67 mm case length: Guns with a chamber length of 2 1/2" or longer, nitro proofed to a service pressure of 3 tons per square inch (850 kg per square cm.)"
Recoil is not necessarily related to pressure, but in light of the age of the wood of classic American and British SxSs, it seems reasonable to limit pressure to 8,500-9,000 psi, and several commercial loads are available from 5,000- 7,000 psi
http://www.doublegunshop.com/forums/ubbt...ge=11#Post31805