3 1/4 tons refers to service pressure, not proof pressure. One article I have, which credits the Birmingham Proof House with "much of the technical data", says that 3 1/4 tons converts to 587 bars, and that the proof pressure for those guns was 939 bars (both measurements crusher rather than transducer). The significant gap between service and proof pressures in that article, however, looks suspicious to me. In a direct communication from the Birmingham Proof House concerning the post-tons measurement guns (850 crusher bars standard proof), the figures given are 740 bars psi service and 960 bars psi proof. Note that those figures are much closer together than on the "tons"-measured guns, which is the source of my suspicion about their accuracy.
In any case . . . Again, direct from the Birmingham Proof House, the service pressure for standard proof guns under the more recent "bars" measurement, converted to psi, is 10,730. That's below the SAAMI standard of 11,500 psi, but not by a whole lot. I would not think, from the numbers listed, that the 3 1/4 ton guns had a higher service pressure than the 850 bars guns, which in turn means that standard American 2 3/4" factory loads MIGHT produce somewhat greater pressure than that for which the British guns were designed.
Caveat trigger puller, I guess.
