The problem with pressure figures from US loads is that they do not remain constant from lot to lot. American ammo makers load to a velocity constant, not to a pressure constant. They don't always use the same powder, so pressure will vary. But as long as the pressure does not exceed 11,500 psi for a 12 bore, they're good to go. But if they say the velocity is 1330 fps, for example, that's going to be pretty close.
The other issue that needs to be emphasized here is that the "tons" marks were service pressures, not proof pressures. They were replaced by bars (850 bar standard, 1200 bar superior or magnum in the case of 12ga guns), and those were proof pressures. HOWEVER . . . in both cases, those pressures were measured with lead crushers, not piezo-electronic transducers, and therefore cannot be converted directly to psi because those are transducer-derived numbers. In the case of the bar proofmarks (or kg on other foreign guns), those CAN be converted, if you multiply x 14.5. HOWEVER, the result--if you start with bars as measured by crushers--will be LUP and not psi. More recent guns from Spain, for example, do show proof pressure figures in excess of 1300 kg or bars. Those are transducer measurements, and can be converted to psi--or at least reasonably close (there is apparently some variation in their transducer measurements to ours)--with the x 14.5 formula.
Complicated subject. I have never seen anything confirming proof pressure on pre-1925 British guns. However, a retired proofmaster confirmed that the proof loads used under the 1925 rules were essentially the same as those used under the 1954 rules, when they were marking with "tons" figures. And since he was proofmaster before the changeover to the 1954 rules, and for some time after, I think he's a pretty reliable source on that score.
Last edited by L. Brown; 02/02/14 06:40 PM.
