This is the question which never dies. These are maximum service pressures we are looking at, based on proof pressures for current, 1989-date proofed guns?
Proof pressures and service loads are complex for a number of reasons. We are dealing with multiple proof periods and first we need to figure out when your gun was proofed, to understand what pressure it was proofed for. During some periods only chambers and shot load were marked, or tons per square inch, or now in bars. British proof markings: The "tons" mark (which is service pressure) was not used until 1954, 1954-1989. 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. Post 1989 they went to bars. I thought standard proof in 1989 and after, was 850 bars with a service pressure of 650 bars, not 740 bars. As per Larry Brown "850 bar proof/650 bar service pressure guns (were) (crusher figures). Got it straight from the Birmingham proofhouse that the 650 bar figure is actually 740 bar transducer, which is the source of that 10,730 psi service pressure" I guess I have been over cautious using 650 instead of 740.
Current CIP/ British standard proof is 850 Bar for 2 1/2" and 2 3/4"?
Standard service pressure 2 1/2" and 2 3/4" is 740 bar, 10,733 which is about the old 3 1/2 tons level.
Current CIP Superior proof for 3" is 1200 bar?
Standard service pressure for 3", 1050 bar which is 15,229, which seems very high. SAAMI for 3 1/2" is only 14,000.
Under the 1954-1989 rules I think 3" were proofed at 4 tons squared but some might have been 3 1/2 tons.
Per the 1954 Rules of Proof, here are the equivalent service pressure values:
3 tons--8,938 psi
3 1/4 tons--9,682 psi
3 1/2 tons--10,427 psi
4 tons--11,917 psi
These are all for "upper" service pressure loads, many loads are well under these levels.
These numbers SHOULD apply to guns proved under the 1925 rules, which were stamped only with chamber length and shot charge. Thus, a 2 1/2" chambered gun, 1925 rules of proof, would be equivalent to 3 tons; a 2 3/4" chambered gun to 3 1/4 tons. I was told my 3" was 4 tons and it is so marked.
Bottom line:
2 1/2" & 2 3/4" current proof, (post 1989), 10,733 safe.
3" current proof, (post 1989), 15,229 safe.
2 1/2" & 2 3/4" proof 1954-1989, 3 tons 8,938 safe.
2 3/4" proof 1954-1989, 3 1/4 tons 9,682 safe.
2 3/4" proof 1954-1989, 3 1/2 tons 10,427 safe.
3" proof 1954-1989, 4 tons 11,917 safe.
Pre 1954 you need to figure where your gun proof lines up with 1954 rules.
https://bobp.cip-bobp.org/en/tdcc_public?page=1&cartridge_type_id=7
