Dr. Drew:
I was perusing a box of Gamebore Traditional Game (made in Great Britain) 2 1/2-inch 20 gauge shells today (English 6s, fibre wad) and right on the side of the box it says "These cartridges are suitable for use in: Guns with a chamber length of 2 1/2 (65/67mm) or longer, nitro proofed to a service pressure of 3 tons/sq inch (850 kg/sq cm)". I sorely wish all cartridge makers would list this sort of information for the shells they produce, but no-matter. Is there a "reliable" conversion for tons/sq inch to tensile strength psi? Or....are there some other variables that need to be considered? Just plugging the numbers into an online converter gives me 6000 psi, which is comfortably below my ball-park figure of about an 8,000 psi limit for older guns. I might add that in a 6lb gun, these shells produced some fairly sharp recoil. Nothing terrible, mind you, but comparatively sharper than most of the RST shells I have used.
Lloyd, to add to what Dr. Drew told you:
I have several boxes of Gamebores marked the same as yours. Although the information on those boxes is valuable, it is also somewhat misleading. Mainly because it refers to two values (3 tons/sq inch and 850 kg/sq cm) as if they were equal--but they are not. The 3 tons is a reference to SERVICE pressure, while the 850 kg refers to PROOF pressure. And at the time those values could be found among the proofmarks on a British gun, both of them referred to pressures measured with lead crushers rather than piezo-electronic transducers, which means they cannot be directly converted into psi as we measure it today.
As Drew pointed out, however, the general thrust of the information is accurate. It's telling you that the shells in question are suitable for use in CIP "standard proof" shotguns. Drew gave you the equivalent psi values for service and proof pressure expressed in psi. Especially for service pressure, they are not that much different than our American SAAMI standards: CIP standard for 12ga service pressure is a maximum of 10,730 psi compared to our 11,500 psi. According to the former Birmingham proofmaster, referring to the 1954 rules of proof (the "tons" values): " . . . under the new rules of proof no arm will receive a more severe proof than hitherto." That means that any gun proofed since at least 1925--the proof rules in effect until changed in 1954--is good to go (assuming, of course, a gun in proof and in safe shooting condition) with any modern loads, such as your Gamebores, made for guns proofed under the 1925, 1954 or 1989 rules. The 1989 rules are the ones than required the metric terminology: 850 bar (or kg per sq cm). Guns proofed under the 1925 rules are relatively easy to spot because they will show both chamber length and shot charge: 2 1/2" and 1 1/8 oz, or 2 3/4" and 1 1/4 oz are the ones seen most often. Prior to 1925, shot charge was marked but chamber length was not.
Hope that helps clarify a fairly complicated subject. We and our cousins across the pond, as someone once wrote (G.B. Shaw?) are two peoples separated by a common language.
